Plan B is a human medication that is potential fatal to dogs and other animals - it has NOT been tested for safety in dogs, NO dosing regime has been established and due to the difference in canine and human reproductive systems - i.e. periods vs. heats - even if it did her hormones exactly what it does to a human, it would do nothing to stop or abort a pregnancy.
You have three options:
1. Spay the dog... I promise that the reproductive organs are not where the soul of the animal resides. Dogs don't psychologically need sexual intimacy like people do, domestic canines only care about sex when they are a) a female in heat or b) a male near a female in heat. In wild canines, even sexual receptivity isn't enough as there are complex social rules and environmental factors about who can mate... basically, you are anthropomorphizing and it is causing you and your dog no end of grief.
2. Confine the dog properly when she is in heat. This should be a big no-brainer... it takes two dogs to make puppies and she is only in heat for 3 weeks twice a year. Keep her crated, walk her on a leash, and keep the males away from her. Voila, just like that no more puppies.
3. Give the dog away to a responsible home or rescue. If you don't want to properly care for the dog, then find her a home that will.