Don't use your teeth...
First, do understand that zoanthids contain an extremely deadly toxin (as in deadly to you and most any other living thing around your size or smaller).
If you can use bone cutters or something of the like, it's easiest to nip off some of the rock that they're on and then glue that onto a frag plug. Depending on the size of the colony, you can nip off a bit and then gently pull the mat up and get whatever you want off to frag.
However, I'm no expert and there are some here who are VERY familiar with zoanthid fragging (cough - MikeW - cough) and perhaps they will comment.