lots of PB blaster and time to let it do its job. I'd spray it down, way half an hour, try it. If it's still bad, spray again, wait another half hour.
what I did with one of mine that seemed seized was use a ratcheting box wrench, put it on the nut and put some vice grips on the bolt (don't wory, smashing part of the threads won't affect the nut coming off) and some brute strength.
If that doesn't work, then torch it, grind it, whatever you need to do. replace with something NON oem so you don't have this problem later
96 Mx6 LSDE R.I.P
93 Mx6 LSZE
94 Miata 1.8T