It might not be nipping. If the betta just doesn't like the guppies then it could just be stressing them, making them more susceptible to illness, not wanting to feed etc.
Every tank is different and every fish has different personalities. But personally i would seperate them.
We had a post on here a few months back where they were having similar problems, they split the tank with an eggcrate divider and the betta jumped the divider to kill the guppy, waited for the guy to see the aftermath before jumping back into its own size.
Keeping other fish with bettas is for your own benefit, not the bettas. Generally they really don't care about having company even if they will tolerate it.