If you want to get sneaky on them without telling them to go fuck themselves you might also start using jumpscripts which are in a special directory. Then put that directory in robots.txt. It has almost the same effect. The Googlebot won't visit the jumpscript and will never see the sponsors link. All it will see is a link to a internal page on your site which it cannot index due to robots.txt restrictions.
Technically the link would still be dofollow.
