You can't and neither should you want to. It is against ebay policy and if you leave a pos with a negative comment ebay will, at the request of the buyer, remove it and your account will be sanctioned.

If it is because of non payment then you go through the unpaid item process and the buyer gets a strike against their account, much more effective than a red dot that nobody is going to see. Back when sellers could give negs I certainly didn't have the time or the inclination to trawl through the feedback of every bidder and buyer to see if they have a justified negative.