I was looking at fragging mine before putting it in my tank. From the research I did the suggestion was to use a bandsaw, which clearly won't work in your tank. The other less popular possibility was a dremel, which won't work either. I didn't try either.
The reason I mention this is because you probably won't just be able to cut it, you will likely have to saw it.
I took mine out of the frag tank to glue to a rock last night and, even without cutting, it expelled a load of slime. I would think that sawing it in the tank, if you tried with a hand saw, might be pretty messy.
I'm interested in any expert answers as well. Maybe there might be something (another coral?) that you can put in its path to control its growth, but I'm not sure what.