Wamp might have the only semi-safe way of doing this.
Otherwise I would think it would be extremely difficult to break that hard base without damaging the soft part severely, stressing the coral or killing it.
Not sure why you bought it if it was too big, but maybe you could take it back for a smaller one?
If you have to pull out the hammer, dremmel, plyers, whatever, at least have some Tech-D Coral Dip handy to help prevent disease.
Sorry, hammer corals are my favorite.
