If you're talking about it in a "romantic" context (I put that in quotes, as any person who thinks people can be "too nice" is unmistakably not romantic), have a look at this:
YouTube - The rejection of "you're too nice of a guy" - a prospective (part 1)‏
If someone told me "be less nice", I would be tempted to tell them to "be less around me".
Though if I wanted to be more polite, I would ask them why they thought that way, to get a better understanding of their mindset. I would further ask whether they envisioned their ideal relationship with people as being based on kindness and mutual aid, and if not, what other principles their ideal relationship would be based on. I would point out that, according to common sense, people are less happy with each other when they are less nice to each other, then ask him/her to be nicer to me, so that we could be happier with each other. Since he/she wouldn't want that, I would then curtly explain that I don't want to bother building half-hearted relationships and leave it there.