rap on it mildly to moderately with a mallet-not a hammer...often the corrosion caused by non-distilled water in the coolant will keep if from coming off as easily as it should.
if you can remove the studs, that would be even better, then you can just tap it straight up or down and it would pop right off
...you could also score the area between the mating surfaces with a putty knife or common screwdriver, that could help you out a little. if you can manage to get one slightly wedged in between the spacer and the intake body, just "walk" it around slowly, and it should help loosen up the crud that is no doubt holding your spacer hostage...the only reason i don't recommend using a razor blade is for the inherent slicing-your-finger-off factor