Yes they are annoying but....

Use the Unpaid Item Case correctly and have your own auto-blocks in place and they don't botther you too much.

But if you go leaving negative comments within POS FB as you have done you only expose yourself to getting a strike on your own account for breach of policy. If this buyer complains to ebay about your negative comments you will find yourself in trouble and the FB comment you left them will be removed.

Don't let them get under your skin as you will just bring yourself down and they get away scott free.