If 50% of users turn off signature viewing, then the signature campaign is getting 50% less exposure. So won't they want to pay less?
A small decrement maybe there but due to the competition, they will pay an amount which isn't disappointing.
Why should users be forced to view ads they don't want to see? We already allow forum ads to be disabled, I don't see you being concerned about that though.
Users can also disable their signature.
-MZ