Some people like to believe they have a chance until they realize it is impossible. A 5% chance is still a chance. As long as they believe that there is the possibility of getting what they want, they will keep trying. You can't leave it vague and expect them to just go "Well it is likely that she isn't interested in me, even though I'm not 100% sure I'll just stop trying." That isn't how they operate. They aren't looking for signals at all. They are looking for a definitive answer, so there is no signal you can send to get them to back off because signals are usually open to interpretation. That is what differs between a signal and a direct response after all.

Unfortunately you are in a lose-lose situation because there are some people that want a direct response and some people that will blow up on you for a direct response as you have already said. I don't think there is any way of telling which it is before it happens though. Ultimately their feelings are their own problem, so just do what is best for you and stop worrying about how to hurt them the least because they certainly aren't giving you that much consideration.