The easiest solution would be to ban account sales officially.
If not on bitcointalk,the account sales could easily take place on other places.I'm sure that already exists.
BAN SIGNATURES FOR FORUM MEMBERS WHO HAVE NOT DONATED TO THE FORUM.
What ? lol You might have donated to the forum when the bitcoin prices was what $10 ?
Complete illogical statement.
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.
If they had 2
BTC to donate,why would they post garbage around the forum for few Satoshis ?

Afterall,2
BTC is a huge amount for a campaigner.What you implied is,allow only rich to earn more money on the forum.