Fortunately, these pretenders can be spotted a mile away, because despite what you may have heard, libertarianism is not complicated. The best working definition of a libertarian is someone who judges everything against the Zero Aggression Principle, or ZAP*:
A libertarian is a person who believes that no one has the right, under any circumstances, to initiate force against another human being for any reason whatever; nor will a libertarian advocate the initiation of force, or delegate it to anyone else.
That nails it. It is the initiation of force that is the important detail; a defensive return of force is entirely permissible under the ZAP.
And this is what exposes pretenders for what they are. Anyone who would support using the power of the state to coerce, force, or compel others to do their arbitrary bidding, no matter how noble or agreeable the topic may be to anyone, is no libertarian.
Thing is, anything that uses tax dollars is immediately in violation, isn't it? Throughout history, taxation has always been arbitrarily legitimized theft, enforced at the barrels of the guns of the guys with the tin stars. (For Pete's sake, it's a primary reason we fought the Revolutionary War to begin with.) "Don't think government is all about compulsion and force? Try not paying your taxes." "Governments are the only vendors that don't let customers walk away." And so on. It's no less initiated aggression than the mugging it so precisely resembles, and it is used to fund all the other, more obvious aggressions that we mostly preoccupy ourselves with.
So, the next time you see a "libertarian" championing a tax-funded government program--any such program--you'll understand why some of us start chuffing and rolling our eyes. It's not because we're snobs for purity... It's that nobody likes to be impersonated that badly.
*Thanks to L. Neil Smith for popularizing the ZAP, and certainly for bringing it to my attention. The elegance of the wording is modern, but the idea is pretty much straight out of the book of Jeffersonian liberalism.