There is no known birth control that is commercially available for dogs that I've ever heard of. The only things you can do is keep a constant eye on your dog and NEVER leave it unsupervised when it is in heat. Or get it spayed.
We used to use a product acalled Ovaban years ago, but what that does is to stop the cycle and put it off until you stop the pills. I would not reccommend it as we used it on cats and it would eventually cause hormone imbalances which would necessitate spaying. I'm not even sure it is still available. We have a number of clients who bring their girls in heat in to be boarded. They are in an indoor outdoor kennel behind and under chainlink, on cement and accidents don't happen. These are either purebred dogs that people do not want to be bred for whatever reason, or dogs that cannot be spayed due to health concerns. I would reccomend that if you believe your girl is good enough to be used as breeding stock, if she is worth spending the money to have her x-rayed, OFA'd, CERF'd, screened for VWD or any other potential genetic disorder, as well as screened for brucillosis, she is worth spending the money for 10 days board. Research the local kennels and see what is available, tour them, all kennels are not created equal, pick one you feel comfortable with.
Candy if I were you I would just let her be and keep a eye on her when the heat comes. I made doggy diapers for my female . They are velcroed on to a nice snug fit so nothing can get to her and it helps with the mess too. Also when she is outside I go out with her and when her business is done I put it right back on her and she can still stay out and play with out worring about a unwanted pregnancy!! I have about a dozen paires and I change them about 4 times throughout the day . They are washable too.
i know of a vet that uses the depo provera injection for dogs. how safe or ethical that is i do not know. the only birth control i know of that is 100 percent effective is spaying. other than that you can go to the pet store and buy special panties for dogs in heat. they are made special for situatuions like yours. they generally cannot urinate with them on so you will need to take them off for her to go to the bathroom .
I believe in Europe there is some type of birth control. It is not available here. But I have heard (Vet magazine)of some Veterinarians are looking to develope an injectable birth control for every 6 months.