Blocking accounts is surely backwards logic. Surely not blocking accounts and grassing up any bastard the posts or reads their idilogical bull shit would be a far better proposition. And make it public knowledge that it's how you run your service.(I.e. the Gov don't have to ask for the data you just offer it up proactively) That way terrorists and sympathisers are unlikely to use it unless they want an army of angry looking military personnel coming to their house with some guns to have a word. May have to block access from TOR/anonymous networks though.
Could be wrong (and I'm sure there must be a flaw in the plan somewhere), but that has to be easier than playing wack-a-mole and may lead to actually catching some scumbags in the act.