It seems to me that the majority of objections to Universally Preferable Behavior that I've seen, stem from a belief that morality MUST have a crapload of baggage attached. People can't believe that it's really just "Don't hit, don't steal, use your words." Universalized. Force may be used to stop hitting and stealing, and failing to persuade people to help you is, de facto, ostracism.
Defensive force must be proportional, which, I'm afraid, can only be defined by experiment.