There's not much you can do other than try to come up with other ways to make money fro your forum. Or just put a slight message on your site saying how ads help pay the bills blah blah blah and to ask your users to bypass just your site when using an ad blocker
I don't think there is much you can do. Possibly banners will go the way of dinosaurs at some point. In that case, you would have to find other creative methods. One might involve simply making a section of the forum an ad. For instance, one forum could be a link forum that goes to a sponsor.