I get asked this quite often:
Why does my male neutered dog still hump?
Humping is a dominance behavior, not a sexual one. He is expressing dominance over the other dogs. When you catch him in the act, simply take him and separate him from the other dogs and the people in the house – isolate him for about 15 minutes. This is what dogs in a pack do when a member behaves rudely. It will take some time, but he’ll get the idea that humping gets him isolated and he will stop