How can I remove a granite countertop from a wooden vanity?
You'll have to get it started somewhere, but a thin wire wrapped on a handle at each end (as for cutting cheese) will normally cut things off if glued with something reasonable (ie, silicone or similar.)

If they epoxied it in place, it's going to be much more difficult. Heat would be my first approach in that case, hoping the stuff softens, or turns out to be hot glue rather than epoxy.