I have found that an email to the violating party along with a bcc to the affiliate program as a heads up works sometimes. There have been times that the program will contact them and if the situation is not corrected, they close the affiliate's account. If the violating party does not correct the violation, then I file a DCMA form with the violators hosting service.
The general result is the violators host contacts them and if they do not comply they shut the website off. That usually gets the person's attention rather fast.
In cases where the webhost ignores the violation, I fax over a DCMA form to Google and the other major search engines. They will impose a penalty and remove the violator from the search results.
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"People who are unable to motivate themselves must be content with mediocrity." ~Andrew Carnegie~