Put their ID on your blocked bidder list:
http://offer.ebay.co.uk/ws/eBayISAPI.dll?BidderBlockLogin
and then tick "Don't allow blocked buyers to contact me" at the bottom of this:
http://offer.ebay.co.uk/ws/eBayISAPI.dll?BuyerBlockPreferences
That only works if they have not been in a recent transaction with you.
