None of my sons ever did this, but ds #1 did have a little friend in our playgroup when he was about 18 months old that did this same thing. My memory is a bit fuzzy on this (ds #1 is now almost 12) but it seems that the boy kinda got the message that his friends did not enjoy his invasion of their personal space (most of them pushed him away, some rather forcefully) when he would begin "humping" them or their toys. I do remember that a few of the moms were very concerned about this and there were a few conversations about how to approach the mom about it. I don't know if they ever did say anything to her or not... I was not concerned about it and felt that the kids were doing a good job of handling it themselves.