in the same way adsense publishers can "ban" selected sites from showing up, can adsense publishers prevent certain sites that have adsense on them from displaying an ad they placed?
Did you mean AdWords there? If so then No, all we AdWords users can do is to either choose to indeed show ads on 'content network partner' websites OR search only. Not per-site.
If you are speaking of AdSense, then yes, you login, go to add settings -> URL filter -> Add/edit sites and add only the website url that you do not want displayed on your website. If you are speaking of AdWords, then no, I do not think so.
Did you mean AdWords there? If so then No, all we AdWords users can do is to either choose to indeed show ads on 'content network partner' websites OR search only. Not per-site.