Bellhousing nut and stud query

Hello,

just trying trying to remove the bellhousing on my series 3 gearbox; have got to removing the last large nut (1 of 4) and after loosing it off i have ended up with the nut still tight on the stud, the stud turning freely but not winding out! what might be going on? ive tried pulling it out with mole grips but it wont pull, yet it turns freely?
any help would be appreciated.

if its one near the starter it will be a bolt, others threads have stripped ,it will come out when you split box from engne with help from some leverage

Thanks James,my incompetence reaches new levels; i didnt realise it was a bolt through to the outside of the rear of the bellhousing.
Maybe i had better read the manual more closely