Safety first
Like Paco, I first learned to say no in the military. I've lost count of the number of Field and General grade officers I've told that I couldn't do one thing or another because it's not safe. Once you trot that word out, everyone starts to have second thoughts. In my last assignment, I got in hot water a couple of times because I refused to fly overweight (we still did the mission, but with half as much cargo as they originally wanted to take) and refused to fly overwater without the required safety equipment onboard. In both instances, I was exonerated, but my name was mud and this led to me finally deciding to leave the service, a decision I have always regreted. Sometimes the result of saying no isn't that great.
I've been flying in the offshore business for years now and find the same rule applies, tell them it's not safe and they will back down immediately. As Chief Pilot, I would never ask a pilot to do something I wouldn't do myself, and if I have to duke it out with a client over a pilot's decision to turn down a request, so be it. I just tell them it's not safe and everything changes. To their credit, all of our past and present customers are really good about accepting the pilot in command's decision, and normally don't question it.
If reason will not prevail and you want to break the aircraft in a hurry, pull a circuit breaker or two. That will "fix" things in a hurry. The safety of the aircraft and it's passengers is ultimately the pilot in command's responsibility, but I've always been more inclined to worry about that guy in the front seat!