The main thing is ddosing them. Another thing is you can find their address and ISP info so you can call their ISP and report him for all kinds of shit.
There is alot more, but that takes knowledge and patience. I suppose you could port scan them to see if they have any particular ports open that are vulnerable to all kinds of windows exploits.
You can pretty much ask google any type of question you want in this department and get a solid answer, you just need to word it right.