Get the girls tubes tied just like human females. Find a Vet that will do the operation. Rather than spaying the girls, just remove a section below the ovaries. Girls still go into heat. Males have their fun....no puppies. Don't want to do the females than do the male. Just like a Vasectomy in men. Males have their fun with the females. No puppies. Riskier since other males can still jump the fence to get to the females.
Oh yeah. Be a responsible pet owner. Pick up the poop, keep the female away when in heat.