I believe commercial content is allowed in signatures. I don't know how that translates with regard to referral links, though.
There is a Moderator Signature Committee that reviews signatures for questions of appropriateness. A good starting point may be contacting the Community Director, SanDiego1K, for further guidance on how to submit signatures for review.
If it's a contentious issue, TalkBoard can look at revisiting the policy, but as far as enforcement, that is indeed a moderator/Community Director thing.