Easiest way is to disable all signatures for all users and that will repel 99% of the signature spammers. No incentive = no hollow forced posts containing 100 of characters saying basically nothing. Actually I am kinda curious how the forum would look like if no signatures were allowed. I have stopped reading threads past the head post because I know what is waiting for me underneath. How much traffic would the forum stand to lose if there was no more incentive for posting? 50% or more?