You can also try using tweezers to pull the zoanthids from the rocks. Be prepared for a few casulties though. Some zoas will come off real easy, others will not. If these encroaching zoas are one of your most prized aquisitions, you might want to try something else, but if it's a common variety, go for it.
Another method is to use some epoxy and build a barrier/wall in between the two. Once the zoas start to overgrow the barrier, just pop it off the rocks and build another one. HTH.