Affiliate Merchants That Dont Pay

What action should I take on affiliate merchants that dont pay commissions owed? I currently have two merchants that are past due with their payments for my promotions... and "past due" would be a gross understatement.

One merchant owes me $602. The payout is supposed to be sent monthly (assuming minimum is met, which it has been consistently)... yet I havent received a check from this merchant since the end of March 2005 (in 8+ months). That payment was my very first commission check, and was $175. I have earned a total of $777 in commission from this merchant - leaving an outstanding balance of $602.

I corresponded with the owner/manager several times in August, September and October 2005 - both by email and by phone. Each time he assured me he was going to handle it personally ("walk the check out the door"). I got an apology and an explanation that they were transferring to a new service to manage their program, etc - and assurance it would be taken care of.

December 1st - I call the toll free number I usually reach this affiliate manager at... and the number is "no longer in service". I sent an email... but at this point, I am not sure what course of action to take next?

I do have one other merchant that owes me several hundred. I am sure whatever advice you can offer me on the 1st one will apply to the second one as well.

For me, it's the principle of the matter. These guys owe me a chunk of money, and I intend to make sure they own up to their end of the agreement. In addition, I have advertising expenses and time put into those promotions - so I'd like to see that money back, of course.

A thousands bucks is a thousand bucks. Your thoughts?