Welcoming strangers, protecting people’s boundaries, and not unnecessarily hurting people’s feelings are all important values for a dance community to have. Any norm we agree on is going to involve some trade-offs between these, but some norms are going to be better optimized for promoting *all* of these values. So here is the norm I recommend:
If someone has a reputation for being boundary violating or if you have seen them behave poorly, you can say no to a dance with them in an assertive and dismissive way. It’s important that people set firm boundaries in such cases. (Also, there should be better tools for ostracizing people if there is evidence of them behaving inappropriately.)
If you know nothing about a person, you should say something sentence-length like, “No, but thank you for asking.” It’s important that you convey that you care about their feelings. If they ask you to dance again soon after, you can increase your aggressiveness and dismissiveness until they get the hint.
If *you* ask someone to dance and they say, “No, but thank you for asking,” you should err on the side of ‘it was decently likely that they were just being polite,’ and not ask them again for a while—excepting cases where they strongly signal that they would like to dance with you.
Also, there should be a norm that you are *not* allowed to complain about someone saying no to you in a dismissive way. This could easily be a tool that jerks would use to deter people from appropriately protecting their boundaries. A suggestion that everyone be nicer, in general, is not synonymous with the claim that you are allowed to complain when someone specific is not nice to you. As an analogy: If I suggested we all give to charity more, this would not be license for any specific charity to say, “Hey, why didn’t you give to me?”
A norm in dance communities I’ve been in is if someone who you don’t know asks you to dance, there is nothing wrong with saying simply “No.” Also, there is nothing wrong with saying “No,” and then immediately accepting a dance from someone right next to them. If someone asked you to dance recently and isn’t taking the hint, this is good advice. If you have evidence that someone is a jerk, this is good advice.
But it is unnecessarily rude and unwelcoming to *default* to a monosyllabic dismissal when you know nothing about a person. Dismissive behavior should not be our default mode.