Do I recall you having a simular problem be fore? Could this be a continuation?
I don't have a biting problem with Merlin, He just hates the ups guy , I have tried everything It's not just the same guy. So to prevent any harm to either the guy or Merlin I just keep him in the bedroom when he arrives (almost evey day). It's the major problem he has. I'm very lucky he was socailized well as a pup by the whole family.
Good luck on the problem. Maybe you should get the book on how people and animals interact. I just finished reading. The title, Pack of two.