I'm trying to remove the alternator, first time doing so, but the 12mm nut on the rear of the alternator appears to be spinning without loosening. At the risk of asking a stupid question - it's not supposed to do that right?! I've dropped an image below for reference.

Are there any tricks to doing this properly? I've tried anchoring the bolt in place with long-nosed pliers and rotating the nut but to no avail. In all I spent a good 2 hours spinning this thing with all sorts of different tools, utilizing many expletives in the process.

 

Are you trying to keep that insulation piece behind the nut? If not, I've used a vise grip and clamped the bejeebers out of something like that. Then, I hope that it stops the bolt from spinning.

 

The insulation piece is necessary to hold the rubber sheath onto the..thing. If it breaks I'll be in a world of pain no?

 

Did you try applying a sideways torque to bolt, via socket that is simultaneously trying to loosen?

Essentially jamb bolt sideways to immobilize it.

 

Not yet, but I will try tomorrow morning.

 

Yes, if you have no replacement part, don't use the vise-grip.

You may need to remove the alternator from the car. This will make accessing the hidden bolt head easier.

 

Have you thought the hammer and chisel method may work ? if so, a shop may be best

 

An impact gun sometimes works.

 

Sounds like the person before you stripped the nut, here is what i would do

disconnect the battery
unbolt the alternator
clip the wire with a pair of dikes or lineman pliers
install a new terminal on the wire
problem solved