"If you do not wish to dance with a person who asks you, then you should reject that dance, and no further explanation is necessary. (Unless you desire to elaborate, of course.)"
Swungover adds that many dancers add "I’m sitting this one out" after saying no. This is a problem because:
"First off, they will probably ask again, later, and so you are simply prolonging the problem rather than solving it. Secondly, your personal safety — whether physical or mental — is much more important than social graces. The rejected partner may ask why, in which case you have the opportunity to give them honest feedback on their behavior. “Well, to be honest, in the past…” (Depending on the behavior, you may want to give them feedback on it regardless of whether they ask for it or not.)"
There are two important ideas here. First, your mental and physical well being is far more important than being nice. You have no obligation to be polite when someone is making you feel unsafe.
Second, being honest gives you a chance to improve the situation. "To be honest, last time we danced my shoulder was really sore afterwards. Can we do a gentler dance this time?"
Being able to give (and receive) this kind of feedback is very important to improving the overall safety of a dance scene. I'll cover it in detail in a future post. For now, here's one of my favorite dance videos: