On a wrestling forum I go on, they have this message in the rules.
SPAMMERS BEWARE:
If you sign up and are planning on spamming the boards we will not only ban your email and IP Address, we will also trace your IP and report to your ISP, we will also pass your name, email & IP address to over 200 forums and websites and have you banned from using those sites too.
So Spammers be warned!
This would stop trolls altogether.