That's a way to try to solve the problem, but is not only about the signature campaigns. Someone could use AI to grow up accounts and then sell them or use them in an organic way once they hit legendary status, that's why i think is a forum problem and not only a signatures problem.
show me one account like that. i bet its pretty rare. so i don't think that's the main problem. plus who is buying established accounts? no one has ever offered me anything to buy my account from ME. 
I'm not even sure why they would do that. unless they are looking to just spam the forum. which wont last too long.