I have and I did the hour and signed the log book as per the regs. I have also refused to get in an aircraft with a pilot. The only thing you can do is refuse to do the flight if you believe that your name will be tarnished.
I have no training or authorisation to examine the standard of SEP pilots as a FI.
And I wouldn't be paying for the aircraft either.
As you say genghis the vast vast majority of pilots know that things didn't work out and they book themselves in for training. But even if they do I always sign the logbook and do the hour because that is what the contract is for doing the flight.
There is nothing to stop a pilot booking a 1 hour trial flight with an instrutor then producing a log book afterwards and requesting the signature. And their is no reason for the instrutor to refuse to sign it.