I disagree.
Spray painting & arson are crimes which do not demand a great deal of intestinal fortitude. Their actions are staged for the media as much as they're for destructive purposes.
Back when I was in college, earthfirst! (a forerunner of ALF / ELF) was allowed to sell their "monkey wrenching manual" in the student union. The type of people normally grouped around the table were the typical pansy leftists.
I don't think much has changed.
I have no doubt that there are some hardcore types at the nucleus, but most of the followers are involved for lesser reasons.
Start taking the fun out of it and those on the periphery will realize the error of their ways or just drift away.