Yes even spayed and neutered dogs can sometimes hump for sexual reasons.
There are other reasons why dogs hump as well. Some dogs do it when they are excited,bored, or anxious. It's a behavior that seems to be calming for some dogs. Humping can be part of play behavior (it's very common for puppies to hump their litter mates during play, for example) and also to show dominance.
My male dog is 3 years old, and was neutered when he was 7 months. He has one specific toy that he likes to hump even though he is neutered. He likes to hump the toy right before bed - we think it might be a calming behavior for him, that it helps him calm down from the day and get ready to relax and go to sleep. It's a little weird but it doesn't hurt anyone so we usually just let him do it. If it bothers you, you should be able to train your dog to stop or you could always take the stuffed animals away.