Sounds like the gearbox mount to the right of the engine bay, as you look into it, not the rear engine mount as described above.
If the bolt is spinning then the nut underneath has parted company with the rest of the mount.
If you lift the gearbox slightly does the mount separate enough to get a spanner/grips on the now free nut? If not, release the sub-frame mounting bolts on the mount, support the engine and 'box and then release the bolts holding the mount to the gearbox - attend to the errant mount outside the car.