You can put a rel="nofollow" attribute in the anchor tags for the banners. Google says they will not pass PageRank in that case.
That might violate the terms of those sites that required a link in exchange for a listing. Check to be sure before you do it because you might be seen as a cheater if you add a nofollow, and lose some of your most valuable links.
A healthier way to address concerns about diluting PR would be to increase the number of links on your home page that lead into your own site.