Unforutanely, you can't, however if you have achieved such rapid growth that it has become an issue for you, that's a good thing my friend and congrats.:thumbsup:
Zoanthids reproduce both sexually and asexually with the later being the most dominate form of reproduction. The very best thing you can do is isolate your zoanthid colonies by not allowing them to touch other rocks in/on your reef. All you need is about a 3/16" to a 1/4" gap and they will cover that rock alone until the touch the substrate. This small gap has worked wonders for me over the years if I wanted to isolate growth to one rock. Place small chips of rock if you'd like around the base of this rock and they will migrate onto these small rock chips and you can then sell, trade etc these frags.
Hope this helps my friend and the best of luck with your tank.
Mucho Reef
PS. You have to watch those polyps though as they can extend over to those nearby rocks even with spacing and take off like you wouldn't believe.