How can I control a young female dog from humping every dog in sight no matter the size?

I watched a serial humper for a 5 day stay a while back. He was an old guy who was particularly fond of puppies. He did give up on both of my dogs after a couple tries, because both were firm with him from the start, but he was obsessive about humping two other young dogs. Over the course of the stay, I made a lot of progress teaching him to disengage before he got too intense. In my case, I used the cue "walk away, George," to let him know that he needed to find something else to do. I started by body blocking him while giving him the cue, or giving the cue as I was removing him from the other dog as gently as possible and then blocking him until he gave up. After consistent use over a couple days, I was able to catch him before he mounted most of the time, and I rarely had to physically disengage him anymore even if he did start humping. I could more easily get his attention and give the cue from greater and greater distances when I saw him get that look in his eye, and he'd give me this "fiiiiiiine, you caught me," look as he turned away from the other dog. It took a lot of my attention, but he did make a ton of progress in the few days he was with me.

Is she spayed? Lets say she IS spayed, mounting is done for many reasons. Try to determine if she is bored, stressed or anxious. Perhaps she is just very excited. How to stop it: I love that you are giving her the dispensing food toy, that will surely keep her busy. I have had success with just a plain firm correction. The key is to see it coming before it happens and correct it. Don't wait until it happens; do it when you see her getting ready. They show signs before mounting. First give a correction, then give her a time out. Be consistent and you will see that she will correlate that the mounting will cause a correction and a time out and it's something she should not be doing. Every dog is different, I hope this helps.