@christian: If the visits are actually reaching your site (unlike the ones in the link I posted above), then you can use Advanced Blocking, as long as there is something consistent in the referers. For example, if you have host1.badsite.com and host2.badsite.com, you can block “*.badsite.com”.
More details on advanced blocking are available here:
How to block a referer
If you need more information, can you make a separate post from the main forum page? (here) The www.ads-software.com forum rules ask us to keep each person’s questions separate, and that also makes it easier to be sure we can read each new post. Thanks!
-Matt R