I admit calling it spam is a bit excessive, since it's just one thread, but that doesn't make it any better. Most of the forums I used to frequent would have banned the poster and locked the thread within minutes.
Anyway, I'm against locking threads, and this discussion has nothing to do with moderating. Blatant, unwarranted solicitations aren't against the rules (yet...?). They're just incredibly annoying on so many levels -- always have been. That's why so many businesses don't allow solicitors.
A slightly more subversive, but generally more interesting and effective way is to start a discussion about what forums you visit and why, or why you don't visit forums. If someone else also frequents your favorite forum, you can do a few cheers for it. Throw in a few anecdotes, and always have a link in your sig... You might generate quite a bit of interest.
And who knows? Maybe you'll find another forum you like even better than the one you were trying to get other people to join.
Oh, and just because I'm an admin here doesn't mean I moderate the forums. I shouldn't and don't. It's such a low-level, involved job with so little tangible reward that I wouldn't even want to.
Besides, I made my point. If it fails to get through, then it must be a problem on the other end.
Carry on.