I guess you are right, eliminating all the signatures will eliminate most of the spam (except of ANN-bumping). But I don't think this is a good solution for the forum. Because it's necessary for the forum to be an important platform where third parties are willing to place their ads, because Bitcointalk has also costs. And the signature campaigns bring clicks. Bitcointalk needs these clicks to be attractive for placing an ad of third parties.
No one will pay for an ad on a website, that has no visibility (clicks).
That's not an issue. BitcoinTalk has at least 500
BTC being held by OgNasty (escrow).
I don't think money is an issue for theymos. But spam on the other side? Hell yeah. People are so ungrateful for what they have in this forum that they just want more and more, thus they keep spamming the forum.
What do you think will happen if the spam doesn't stop and theymos just give up on putting passive measures in place? Right now, merit was only created because he likes how people from other places of the world can get a few bucks in here while contributing with the forum. But if this doesn't stop, there is a huge chance he will just ban all signatures all at once.