That's a real problem and potential hazard. I've seen professionals attach several suction devices to hold on with and then gently pry the mirror away but the possibility exists that it may break and cut someone. The wall will have some damage but you should be able to repair with joint compound and gentle sanding.

If it is glued to the wall there is going to be some dmage to the wall. The glue is stuck to the wall. The best way might be to bust the mirror and then try and remove the parts that are glued. Be sure to wear gloves and saftey googles no matter how you try this.

Hi! If the wall behind the mirror is drywall, I would cut the drywall around the mirror, and dispose of the mirror and drywall, since removing the glue will damage the drywall anyways. Then you can use a piece of new drywall, tape and mud the seams with joint compound. Then make sure to use a good primer (preferably oil-based) over the new drywall and joint compound.

i'm about to take a big whompin mirror from my wall too--how did your project work out?