However, my favorite solution is this...
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 from 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.
I like the concept behind this idea, although I do think that some of the specifics should be tweaked for it to be something that theymos would consider implementing.
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This would be a very good idea if the amount was lowered (see above).
My basis for the 2 BTC amount is that 50 BTC is for VIP. 20% of the VIP amount is 10 BTC, for Donator. 20% of that would be 2 BTC, for Supporter.