Capone was eventually brought down by the tax evasion charges, because there were too many layers to get to him directly on the other charges. But Ness absolutely put a hurt on his businesses by using those same tactics. Every time Capone hurt one of him men, Ness did the eye for an eye thing.
Do I approve of Mafia tactics? No. Do I approve of Mafia tactics when it applies to terrorists? Yes, absolutely. If someone hurts a member of my family, for example, or is a member of a gang who has open expressed a desire to kill anyone in my family, and I catch them, I will do whatever it takes to get the information I want out of them, in order to protect my family. Same goes for terrorists. Two wrongs don't make a right, but if it's them or me, it's gonna be them. Sorry.