Nothing will be fool proof, but to make things better... don't use banner folder names such as banners/ or ads/ etc. Name them something stupid like goodies/ or funstuff/ etc. Also, shit like this looks for ref codes and other php variables, so unless a sponsor has your ref code built into the links like a directory.. ie
http://www.domain.com/pr-xxxx/... and instead links like
http://www.domain.com/?wmid=xxxx&blah=xxxx&... you should run them through a click tracking script or simple php redirect... ie link to "sponsor.php" which contains the following: <?php header('Location:
http://www.domain.com/?wmid=xxxx&blah=xxxx&... '); ?>