How can i stop somebody from sending me a message 6 months after a sale?

[**peaseblossom**]

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