Some one might have glued them together you could try soaking it in gas.
if that doesn't work put it in the microwave for a couple of minutes then when your at sears buying a new microwave you might see something that you can get it off with
Seriously, remove the studs by double nutting them and unscrew them from the upper manifold. Once the studs are out, a very large rubber mallet, like the ones used to beat hubcaps to death, is the instrument of choice.