Answer
Verulam:
WOT??? The only answer is you can't. You have to keep them COMPLETELY APART from day one of her season even if she's not going to accept him from the start. Once he knows what's going on, even if you have them at opposite ends of the house, living with him will be a nightmare - and when she's in standing heat, her too! He may not lose it for the first week, but he will be biding his time and once she's in standing heat, could well get to her.
If this is going on - do him a favour and get a friend or relative to take him for the 21+ days of her season, and then GET HER SPAYED. Castration now isn't going to do much other than eventually he won't be able to sire puppies. He may well still go through the motions - it's not unknown for castrated males to mount and tie with a b itch who is willing to mate.
Ocimom:
Neuter your dog and spay the female. The female should be confined in the house and prevented from all contact with males.
Dives:
you separate the two and keep them separated for the duration of the heat time.
Elaine:
Spay her. Neuter him. Don't allow the two together.
