Asking users to do the hunting is something that can't hurt, but is not very productive. And now I think that the policy of good first post isn't productive either. Because few bots post at all.
That would work better, but we need a system to monitor when users edit their entries or their sig, not just when they post
Not only a sig. They can edit post body and use any profile fields. On a forum that I moderate sig, bio and homepage are equally popular places to post links.
I think the best implementation would divide user into 3 groups (not visible to anyone but mods/admins), 'most likely a human', 'unknown', 'most likely a bot'. Mods would be capable of moving users back and forth. The first group wouldn't be monitored at all. The third one would be permanently banned and get accounts deleted after some time (so, in case of error, there's a way to protest).
But somebody would have to do the coding and I haven't seen any ready solutions like this. Though I didn't search.
Oh, there's one more thing. Some bots are buggy and enter invalid links. I think such links should be removed too, but I don't see a productive approach.
I haven't seen any spam here for long and I visit the forums several times every day.