Contacting ISPs about forum "abusers" often doesn't work. When I was a mod at Futuremark (then MadOnion), we had particular problems with a regular user - he'd end up getting banned very quickly but would always return under a new account. Finally, it came round to my turn to ban him...cue several new accounts, dozens of abusive PMs and emails, accounts set up using Hotmail accounts made in my own personal name and so on.
Despite repeated contact with the ISP ("we are currently investigating your complaints" they would say), nothing ever came of it. Eventually, it all passed over but I'd had enough and stopped moderating. Half the time, though, the abusive poster isn't the problem - it's the replies that fan the flames that can often cause more grief.