Simply challenge players around the level you should be, explain your rating status up front. Put your circumstance on your profile also.
Beating players well below your previous and deserved rating is not "not nice". It just is what it is. Are there players who are going to go crying home becasue they lost to someone with an 800 rating? Let them cry, and tell them to get a life. This is a game, not our career, and not our family. I will play you if you want, I will lose, and think nothing of it the day after. It is called having perspective in one's life. Do not plan your actions around those who have no perspective and who are thin skinned.