Rock, communication is the key. Contact your customer and try to work with them. I had a negative fb last week reversed because I phoned the customer and dealt with him on a one to one customer service-oriented basis.
Turns out my customer actually left the neg for the wrong seller- he bought the same item from both of us, but the other seller took forever to ship, etc.
It took some time and several emails to help him along in the neg removal process, but it worked. He is very happy with my customer service, and I offered him a great deal on his next purchase for taking the time to work with me on the fb removal.
Give it a shot- what have you got to lose? If he won't give in, then just block him and move on. But if you don't at least try, then you will never know!
Cheers,
AXE