Dealing with link spammers?

I've been working on a project to allow website viewers to talk to each other in real-time based around any particular link .eg. people could be on HN and talk to each other live.

The problem i've noticied is that every now and then we go through a phase of being hammered by spammers who just stream out link after link.

Has anyone experienced this and come up with useable solutions to stop this sort of behaviour.

Currently they are firing out from multiple IP addresses over wide ranges so IP isn't really a good option.

Has anyone got any ideas?

Cheers HNers