Hi UKracer, what a nightmare!!
eBay have now introduced new measures to help sellers from this kind of buyer. I wrote a blog post on it a short time ago:http://barry-wells.com/ebay-helping-sel ... -feedback/
If the buyer is found to be in competition with you eBay can now block all of their accounts, buying and selling, as they're from the same IP address.
Within the post is a link to an eBay help page, set up for this sort of trash buyer.
Just out of interest does this person have any items for sale?
Another little tip: Request a cancelation of transaction for every item that they bid and win, eBay will soon notice the amount and look into it for themselves. Check out the help page it could be your answer.
Best of luck