'pends on the coral and how it is attached. SPS I can usually clip off the plug right at the glue line with side cutters, coral cutters, dremmel or very carefully with a chisel. Softies can typically be scraped off of a smooth surface with a blade. I use an exacto knife with a chisel blade for zoas.
Often the gel superglue is the weak point, and if the coral has not encrusted over it, you can just "pop" the glue loose.
Another way is to use a wet saw, a few of us are buying one this month, they are sweet.
Go to the next PSAS frag class, they do a really nice job of showing the different coral and mounting methods.
-Todd