I am building a forum-type website. I want to have a few ads on it to cover hosting and some basic expenses, and I don't want the site to drown in advertisements. I know adsense allows you to put and not put ads anywhere, but am I allowed to say "Signing up removes all advertisements from the site."
I think there is no problem with adsense if you do this. Many sites show more ads to unregistered users. But I suggest you do contact adsense support to be 100% sure about this.