I have a 1 yr. old Flemish Giant "Thunder"....never had such a horney bunny as him! He too has a stuffed bunny that he thumps all of the time! I had to get him something, so he would quit humping my throw pillows. Yes, sometimes it's hilarious, with the throw pillows he would actually fall onto the floor hanging onto one, lol. Anyway, many times when he is not satisfied with his stuffed girlfriend - he figures he will give me a try, lol. Sometimes I spray him with water, that works alot of times; if nothing else, I at least get a break while he's busy grooming himself from getting wet. I've also shut him in the spare room with the stuffed bunny, leave him in there an hour, sometimes longer, when I let him out, if he's not calmed down, he stays in there longer. Thunder knows this system quite well now. Other times when he sniffs and nibbles at my leg, if I just tell him to "calm down" and pet him, he will lay down at my feet. Which, I've had to do a couple of times while sitting here, he was doing his circling thing, then sniffing my leg. I can't get him fixed right now, so, it's quite challenging!!! I just read that rubbing lavender oil on a rabbits back is supposed to help - so I'll be getting some!
The only other thing that I can say to help others with this problem is, careful what you feed your bunny. I've noticed that certain things that he eats makes him much worse! Parsley and clover especially! Believe it or not, sometimes a few chocolate chips, calms him down. I guess he gets some sort of fix from it, has a sugar crash, lol.
I think that those of us, like myself, who have litter trained rabbits, and only one, may have it worse - since we are their only entertainment! Thunder does not go for being caged, sometimes I put him in a small dog pen, to discipline him. Overall, single bunny owners, well, we need to spend more time petting them, seems to help calm him down.
I let Thunder outside sometimes ( he always comes back within about an hour), that makes a big difference!!! One day when he came back to the door, the neighbors cat was all stretched out on the porch...lol, that cat seemed really relaxed, FUNNY. I wish that I felt comfortable to let him out more often, because that calms down his hormones more than anything. The reason that I don't let him out more; worry about fleas, mites, he won't let me put a collar on him.
Well, good luck to everyone with a horney bunny, especially a buck that is used to "run of the house".