Hi Mark,
Are you having the type of fraud where affiliates pose as someone they are not or claim to own a site that isn't theirs? I had a huge Chinese fraud ring join a ton of networks pretending to be me a couple years ago. They were impersonating a ton of other people and companies and raking in lots of money. It was a fiasco!
Are you calling to verify identities? That's the extra step most networks have had to add.
This thread has lots of good tips and info about handling affiliate fraud.
Fighting Affiliate Fraud - Put Your Detective Hat On