Here is a guaranteed way to eliminate almost all forum spam. Simply don't allow signatures for any forum member. The majority of all posts made on the entire forum are entirely for the purpose of completing bounty signature requirements. Guess what, legendary members can spam too, not just newbies. Eliminate all signatures and you eliminate all spam posts. It takes away the reason to do mass postings every week. However this will never happen, because guess what? The owners of this forum benefit from all of this posting because it increases their number of visitors and therefor ad revenue. The owners probably make a really hefty amount of money from all of their ad revenue. If they take away signatures, ad revenue would drop significantly. So there you have it.
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).