Their IP only really gets banned from creating new accounts on that connection. Their 2/5/100 other accounts they were already farming on that connection will still be fine and free to post especially when there's no admins looking into ban evasion or alts. People can also just go to their local McDonalds/Starbucks/whatever to create new ones or just use a proxy anyway. I think limiting one account to be created per IP should be considered and would cut down on a lot of the abuse as there's only so many connections/proxies in the world and fresh/clean ones would quickly become increasingly difficult to find. Alternatively, as already mentioned (and expanded on below) paying for the signature would help with this issue considerably.

$10-$50 would be too cheap to be effective. I think it would have to be $100+. It won't stop people having multiple accounts completely but it will significantly reduce abuse and the number they can have because nobody is going to pay $100 for their 100 alt accounts. People currently farm hundreds of accounts because it's free and relatively easy to do so right now and that's part of the problem. If signatures are removed from ranks and people have to pay for them then account farming would stop pretty much instantly.  Most people would then only have one or two accounts and just concentrate on those one or two rather than looking to abuse campaigns en mass with multiple ones because it's just not worthwhile or feasible, but as long as people can create an unlimited amount of accounts for free then they will continue to do so.