Neither your vet nor the petstore people are aware that cats 'hump'? LOL...that is kind of weird that they aren't aware of that. Maybe it is cause I had one that did? My RB Tex did it. Sometimes often, sometimes not very often. At first, I was irritated by it, but, then realized it was totally normal. What I suggest you do is find a 'lovey' for your kitty, and allow him to hump that. Don't get mad at him. He can't help it. Kicking him out of your room won't make him change.

Tex seemed to love to do it to this afghan and one particular stuffed bear I had. Maybe there is an item your cat would respond to similarly?