We had a discussion with on-admin-high, and we are going to ask people not to put banners in sig lines anymore.

If you have a website and you want people to know this, just put the URL in your sig line, but not a banner.
We cannot enforce this via the software, so we will contact people via PM whenever we encounter banners in sigs.

SRP earns some money by paid advertisments through the banners, so that we can keep the site going.
Since we chose only to have shaving related advertisments, the earnings aren't that big. If people can also post advertisments for free, then we run the risk of losing those earnings.
We also don't want people posting just to give exposure to their banner.

We haven't had any problems yet because generally, people do the right thing, but we want to prevent problems from happening as more and more people find their way to this place. Since several of our paying advertisers are also members here, losing earnings if paying is optional is a real risk.