I haven't had this situation yet, but if a PPL holder insisted that I sign his logbook, and I didn't want to then I would say something like :-
"OK, I'll sign your logbook to say that you've received an hour training, but I'll also write that I considered the standard to be inadequate - are you sure that you want me to do that?"
Having done that, the decision is defered to the examiner who signs the CofE - they could clearly see that the instructor isn't happy when they check for my signature.
My interpretation of the law at the moment is that the examiner *COULD* then sign the CofE because, strictly, the requirements would have been met - but they can't be forced into producing an autograph if they don't want to.