Forged Email Headers

What do you do if some spamming dickhead decides to forge their headers to make it look like all the crap they’re sending is coming from your domain?

I’ve already put a help ticket in with my hosting provider reporting the problem, and I’m archiving all the bounceback messages. I’m not sure what else there is to do. I don’t really have the time right now to sift through all these messages trying to track the culprit(s) down.