Program owners, what do you do when an affiliate is using the wrong linking code and sending sales but not being creditted for those sales?

I have a guy right now who is sending 10 or so sales a week to Uniform Cocks from one of his sites but i have tried emailing him a couple of times telling him that his linking code is wrong and he hasnt changed it.

This has been happening for about 2 months now and whilst im enjoying the free traffic and sales i really want to find out who this affiliate actually is so that i can make sure he/she is being creditted with the sales they are sending.

I tried the whois infomation thing but they have privacy protection on their domain.

So, what to do, do i just keep accepting the free traffic and sales?

Regards,

Lee