Shadow him for a few days so you can stop him before he gets started. You need to be close enough to him so that you can grab the block before he launches it. As you are taking the block, "We don't throw blocks." Then move him somewhere else.

Sounds like time outs aren't working so I would ditch those all together. If he is doing something, like throwing a block, tell him "we don't throw blocks". If he does it again, he doesn't get to play with the blocks again that day. Period.

Does he play with the other kids? Do they play with him? He may be trying to get attention, too. Search for the positives and praise those. When he does something inappropriate, very plainly "we don't...." and move on. No emotion, no extra attention.

Get parents on board. At 2.5 y he should know better. They need to be reinforcing it there, too.

Good luck! Kids like that are a LOT of work.