Giving this problem a bit of thought, I think one possible way to stop them - or least slow them down, could be for sales management systems to increase seller protection and blacklist refunders, on a network wide basis.
For example, if Ejunkie, WSO Pro, (insert payment & delivery system here), etc, prevented serial refunders from purchasing any product on their network, it might slow them down, or think twice about which products they save their refund allowance for.
So.. let's use WSO Pro in this example (because most of us use it)..
Each buyer could have a limited number, or a set percentage, of allowed refunds. If they exceed that limit, they are blocked from buying any product using WSO Pro.
Or perhaps another way of doing it could be...
If 2 or more sellers click the 'blacklist' button, that buyer is blocked from buying anymore products being sold via that system.
OK, it's not a perfect solution, and people will always find a way a loophole, but it is a solution that existing sales systems could easily implement.
Right now, nothing like this exists.
If a management system had this functionality and enforced it, I'd move my entire business over to it in a heartbeat.