You can first send the user's click to a script that then redirects to the advertiser. This is something a lot of sites do to verify how much traffic they are actually sending to each advertiser.
In addition to implementing the script, you would disallow crawling of the script's directory. Important note: have the disallow rule in place a few days before you change any links. In other words, make sure googlebot gets the new robots.txt first, or else it may get the scripts and analyze them anyway.