It sounds like you already removed the belt. If the belt is on with the tensioner snug, it makes a good counter hold tool itself. Don't worry about damaging the belt - you should replace it while you're in there anyway. Like Andrew says, use a good fitting 6-point socket with a breaker bar and rap it a few good ones with a hammer. I find that the mass of the crankshaft alone (combined with the engine compression) offers enough resistance to crack the bolt loose.