Beware of helpful libertarians.

On the front page of today's Wall Street Journal, a story by Moncia Langley about a financial consultant, Genet JJ McNab, who nabs tax avoiders by posing as a blue-collar libertarian protesting taxes. An excerpt:

When Irwin Schiff, a high-profile promoter of tax avoidance, was sued for back taxes and fraud penalties early this year, his followers ridiculed the government on the Internet. A regular in one online discussion group -- "Libertarian Party Patriot," who described himself as a 44-year-old mechanic from Maine -- sympathized with Mr. Schiff's dilemma and asked about the civil suit and a related criminal case: "Is it true that Irwin is submitting an insanity plea?"

Cindy Neun, Mr. Schiff's girlfriend and co-defendant in the criminal case, responded: "We are sick about having to use this defense. It's ridiculous."

Unbeknownst to Ms. Neun, Patriot was no blue-collar tax protester -- but the assumed identity of Genet "JJ" MacNab, a financial consultant who leads a secret double life as a cybersleuth on tax scams. By day, she analyzes financial plans for rich clients -- and at times provides expert testimony on Capitol Hill. At night, she assumes phony online identities to lure promoters of legally questionable tax shelters into revealing details of schemes that rob the government of billions of dollars in revenue.

In the Schiff case, Ms. MacNab quickly alerted government officials of the e-mail exchange, raising serious doubts about Mr. Schiff's insanity defense. And the co-defendants soon publicly broke up on the same chat room over Ms. Neun's disclosure to Patriot. "I guess [Mr. Schiff] believes that I was his slave for five years just to set him up to destroy his life by writing you," Ms. Neun lamented in one posting.

Ms. MacNab's hobby is roiling the controversial tax-shelter business and the federal agency charged with stopping it. She has proved instrumental in shutting down a variety of illegal tax shelters, from high-end estate-planning techniques to low-end tax-protester scams, according to current and former officials of the Treasury and Justice departments.

By exposing tax-scam artists, she also has revealed Internal Revenue Service shortcomings. Critics say the IRS, known primarily for scrutinizing tax returns, initially was slow to stop the explosive growth of tax shelters and tax protesters fueled by the Internet. Ms. MacNab is "an early-warning device about tax scams," says Sen. Charles Grassley, chairman of the Senate Finance Committee. She "often seems to know about them before the IRS does."

IRS officials agree. "We don't have the resources . . . so we've had to rely on tax professionals like JJ MacNab to be our eyes and ears," says IRS spokesman Terry Lemons. Based partly on Ms. MacNab's input, Pamela Olson, former Treasury assistant secretary for tax policy, who had oversight of the IRS until recently, recommended that the tax agency direct more resources to "looking outward -- at what the tax returns are missing completely."

Still, even those who are ostensibly on Ms. MacNab's side aren't always comfortable with her methods. "She's a cross between a gadfly and a whistleblower," says former IRS Commissioner Donald Alexander. "The IRS likes her and resents her." Adds Jay Adkisson, founder of the Web site Quatloos, a tax-scheme watchdog: "If these actions were done by the IRS, it would be entrapment."

MacNab's husband, Tim Hanford, clearly supports her patriotic activites. In fact, his job puts him in a position to assist the carriage of justice in such cases, given his nine-to-five life as a tax attorney. Does Monsieur MacNab stand to benefit financially from his wife's vigilance, say, in the form of additional clientele? After all, the wife certainly would not pursue the tax evasion schemes of her husband's clients. (Please don't try to tell me that her husband, a TAX lawyer, does not spend the majority of his time assisting clients in avoiding taxes.) Rather than be side-tracked by pessimistic assumptions about human nature and motivation, let's return to the story, where Ms. MacNab vanquishes evil, and, according to the article, almost singlehandedly persuades President Bush to sign a post-September 11th tax shelter prohibtion into law.

Worried about "white supremacists, anarchists, militia and other paranoid and angry types," Ms. MacNab says she assumed the name of a character from a favorite book: Andrew Wiggin, hero of the science-fiction cult hit Enders Game. Taking steps to protect her real Web address, Ms. MacNab joined a few dozen Internet discussion groups.

Nestled in a large sunroom in her home in suburban Washington, with her computer, three cats and tanks of exotic fish, she immersed herself in the intricacies of the tax shelters, as well as the "quirks, personalities, dances and rituals" of its players.

As Wiggin, Ms. MacNab began asking tough questions ("Are you prepared to go to jail for following this program?") and made sarcastic quips ("Aren't you all a bunch of tax cheaters?"). Soon, Wiggin was thrown out of many of the Internet discussion groups.

The setback led to the creation of "Libertarian Party Patriot," with a bogus profile "that fit into their right-wing-leaning groups," Ms. MacNab says. When several bulletin boards asked for specifics, she reinvented herself as a 44-year-old married man in a blue-collar automotive job in Maine. Patriot's hobby? Politics. More important, Patriot demonstrated his affinity with the antitax groups by calling IRS agents "jackboot thugs" and referring to the "evil government persecuting freedom-loving Americans."

Her new identity was soon paying off. In 2001, "Patriot" logged onto a Web site for the Joy Foundation, a tax-protest group. For a fee, Joy provided consumers information on how to have their employers stop withholding taxes. Its Web site argued that paying federal income taxes is "voluntary" and that wages are not income. Patriot received data on the program -- then passed it to the IRS's criminal division. "Justice is interested," an IRS agent e-mailed back.

Eight months later, the Justice Department sued the Joy Foundation, its founder and others to halt what it called a "fraudulent income tax scheme," and sought fines for the promoters. Last year, a federal court permanently enjoined the defendants from promoting and selling their plans. The Web site now says it's "temporarily unavailable."

That April, Ms. MacNab worked with the Senate Finance Committee to put together a hearing called "Taxpayer Beware: Schemes, Scams and Cons." Ms. MacNab was a leading witness, as lawmakers dubbed her the "scam lady."

Ms. MacNab finally had gone public, but she never revealed her use of sleuthing aliases. She embarrassed the IRS by pointing out that most of the scams could be found with a "simple two-hour search on the Internet." Sen. Grassley strongly suggested the tax agency meet with her.

"Her testimony was very helpful," says Mr. Lemons of the IRS. "Tax scams quickly became an emphasis for us in enforcement and public outreach." Indeed, the IRS soon placed on its Web site warnings and technical advice about many of the tax abuses identified by Ms. MacNab.

The following month, Ms. MacNab handed the IRS two more leads. With her phony Internet identities, she had electronically infiltrated violent, antigovernment tax protesters and promoters -- and had found that some of them had identified undercover IRS agents charged with tracking the groups. She alerted the IRS to the names of the groups. Soon after, a Web site displaying what it called "Criminals in Government Clothing" disappeared.

Now, the logical conclusion. Ms. MacNab is no egghead. After all this down-in-the-trenches research on tax evasion, a brilliant idea occurred to hear. In fact, she goes so far as to suggest that this very brilliant idea might be responsible for the next wave of tax evasion scandals. According to the article, MacNab believes that "tax schemes are no longer the wild appeals of the past -- they either appear professional enough to come from high-level law firms and financial-services companies or are in fact from such firms".

Revolutionary. For those who plan to evade their taxes, might I reccommend one tax lawyer with lots of inside information and an excellent metaphysical alibi-- try Mr. Tim Hanford.