[Talisman]

Well, your hacker's not going to leave a street address or blood sample, but they very often do leave their distinctive screen name, symbols, mottos, etc... all over the destroyed site. They might also publish quite alot of detailed information elsewhere while bragging about these activities to hacker friends. They love the attention and notoriety as much or more than any satisfaction they get from revenge.

They're usually sloppy enough to track down quite easily, too... thinking they're beyond reach even if they get caught. So it's worth pursuing.

Good luck.