If it hasn't hurt you by now, it probably isn't going to, so I wouldn't really worry about it. I work with demons and spirits on a regular basis due to my religious beliefs, and the former wouldn't really bother with that sort of a thing; it a waste of time and energy, no offense intended at all. It might be a spirit, or a ghost... probably a younger one, seeing as how you heard "mom" and slapping someone on the knee seems a bit like something a child would do. Personally, I would just tell whatever it is to leave you alone in a polite way, and focus on bolstering up your confidence and personal power. Things usually don't mess with people who are confident and have a certain aura of strength about them. Don't try to "mess" with it, just tell it you would like to be left alone... and then don't take no for an answer. If it continues to act up, treat it as you would a child-- say NO firmly and explain why. It should stop eventually.