BAN SIGNATURES FOR FORUM MEMBERS WHO HAVE NOT DONATED TO THE FORUM.
Create a new level of donator called the "Supporter" that can be reached with a 2 BTC donation to bitcointalk.
Only Supporters, Donators, & VIPs are then allowed to have signatures. Perhaps with each level carrying different restrictions.
This would bring in a bit of revenue for the forums, tax anyone who wants to participate in selling advertising on bitcointalk, and raise the cost of running a signature campaign. All this while reducing the spam, spammers, and those who fund them.
This might have some merit but 2 BTC is too steep. I would like to have a modest signature, I'm not advertising anything, and 2 BTC is way to much for that. Not everyone makes money off their sig.
Having said that, I'd happily give up my signature and avatar if there is a solid permanent solution to this account trade/farming bullshit that's going on right now. I'm just not sure if making it more expensive is the solution as it seems to favor those who can afford it (e.g. advertisers) and might make them spam even more to cover the cost.