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Just say, "Look, I don't want to be in a relationship with you, you're an okay person, but that's not all that it takes for a relationship. There has to be mutual want, and I don't have that want for you, so don't get huffy and try to force something to happen, because the only thing that will, is I'll get tired of it and you."

It either shuts them up about it, or sends them packing, either way you've solved the problem, especially with the second one since obviously they just wanted to be your sex partner and thought they could convince you to go to bed with them.