Ouch! I'm sorry to hear this happened to you. Try emailing PS support, it can't do any harm. They have strict guidelines by which they will remove negs and they don't make exceptions but its worth a try.
One thing I cannot stress enough, please be sure to BLOCK anyone that gives you a neg. Negs mean a transaction went bad. Period. Don't give them a second chance to be vindictive. They can still bid under another ID but at least try to keep them away from your auctions. Here is the URL for bidder management in case you don't have it:
http://pages.ebay.ca/services/buyandsell/biddermanagement.html