I don't know the answer to your question, re chemical spay, but I guess there are many solutions to your situation.
You could have Riley neutered - it would stop him making Bonny pregnant when she comes into season and they get to each other, but there is still no guarantee the humping will stop, as Ollie nathans mum said. They could still mate and tie with no pregnancy produced.
You could leave Riley entire, and have Bonnie spayed before her first season, which again will guarantee against pregnancy, but not necessarily stop the humping.
You could watch them like a hawk, and when Bonny starts to come into season, keep them very safely separated, or if that's not possible, perhaps ask a friend or relative to look after him for the duration of the season.
.........
We recently introduced a male puppy to our house, and had an entire female. I took the decision to have my girl spayed (she is nearly 3). For me, I didn't want the possibilty of an unwanted pregnancy. My girl had matured naturally, and I want the same for our new boy. Her coat has changed, but she is just as lovely as before. He can now go on to mature without any added complications

Those are just my reasons for making the decision that I took. You will have your own, and know what is best for you and any possible scenarios you may find yourself and your dogs in
