Re: “Forum is proudly powered by bbPress”
It’s open source, you can remove it. (I think?)
Still, there are lots of places you can’t put such a statement in. It’s polite to talk about those whose work you benefited from. An about page on your blog/forum with a link would be enough. Or maybe somewhere near where you put your advertising, a link with the logo of bbpress. Unitrusive and can even be pretty if you make a nice banner.