If the person contributes useful posts and continues to do so, I tend to leave sigs alone but I will not allow in-text links or promotional text even with good members. That has worked for me so far.
If I see any post that looks like topical spam (viagra etc get banned period) I ban the user for ten days with a note why and how they can carry a sig without being tagged spammer.
This is a little more time consuming, but I have gained some decent posters that way.