No, he can't sign your names to write-ups for you, unless you authorized him to do so. On the other hand, as practical matter, it doesn't matter: your boss doens't need your signature on write-ups to take disciplinary action against you, up to and including termination. Employment in this country is "employment at will": your employer may terminate you (or do anything less than or short of termination) at any time, for any reason, without your agreement or consent. So he doesn't even really need the write-ups, and him signing them for does not affect his power to discipline you, which was already essentially total.