Not all ads your see are placed by WordPress.com. Every day we experience bloggers here who are seeings ads that are adware contained in shareware ir. browser add-ons and the like.
If the ads you see are found at the end of blog posts then what you are seeing is WordPress.com generated advertising. Look for the About These Ads link to confirm. http://en.wordpress.com/about-these-ads/
WordPress.com does not use text enhance or text link ads or pop-up ads or links in comments. If you are seeing those then that's a browser-ad-on issue, or in some cases, it's embedded into free games and/or toolbars downloaded from the internet. See here: Bloggers: Beware of Adware. http://onecoolsitebloggingtips.com/2012/09/14/bloggers-beware-of-adware/
If what you are seeing whenever you save or update a post or page is an automatically added code/script block at the end of posts and pages that you can't delete then see here: Malware Targets Macs and Windows PCs http://onecoolsitebloggingtips.com/2012/10/02/malware-targets-macs-and-windows-pcs/