disabling or filtering sig ads can reduce post spammers yes, but it will decrease helpful post too even a little.. but for me, maybe the option to filter sig ads can only be enabled and disable based on the forum rank of the user.. the higher the rank the more option/s they will have 
What is the need for 'rank' in disabling/enabling/filtering? Forum is allowing everyone to enable/disable signatures, avatars and ads and also to ignore. So this is option should also be available for everyone regardless of the rank.
Well said..