Unfortunately, you will have to follow this through.

Put the buyer on your blocked bidders list.

http://offer.ebay.com.au/ws/eBayISAPI.dll?bidderblocklogin&hc=1&hm=uk%601d72f%2Bj2b2vi%3C265

Was the buyer new, or established?

If not new, then report it to eBay as well as a hijacked account...it has to be if they are saying their card was used fraudulently...their account must have been too. If not true that should give them a little inconvenience.