Climate compressor, shaft blocked

Good morning.
Golf 4 AC compressor:
Located centrally and rotating with the compressor shaft, just in front of the magnetic coupling, is a nut (M 8, size 14) that has become loose.
I've already energized the magnetic coupling and it engages, but I can't tighten the nut; it just spins the shaft along with it!
How can I tighten it without disassembling or taking anything apart? (How can I lock it in place?)
Who has a ?

Impact wrench when you arrive.

I've also considered using an impact wrench, but there isn't enough space.
Can the air conditioning compressor be disassembled to that extent without opening the system and pivoting it backward to create 15 cm of space?

hi,
Here's an idea I have...
"Energize the magnetic coupling, then the compressor will be supported by the ribbed belt against the stationary motor. Then tighten the nut; the friction in the belt will definitely provide enough force for 30 Nm."
If you ask me, the wave on the toothing will be damaged, because they don't come loose for no reason .

If there used to be any interlocking parts, they are no longer connected.
You turn the nut to rotate the rotor (or shaft) inside the compressor, even though it is energized.
The lock nut also shows signs of corrosion (as the rear wheel sometimes appears rusty brown).
I'm also worried about the integrity of the compressor!

Preliminary conclusion:
Lima and carrier systems expanded.
Compressor loosely attached but still connected.
The mother was fastened with an impact wrench.
It slips slightly when the "M" clutch is open.
It hums quietly when it's powered on.
I also just installed a Lima freewheel.
Everything is much quieter now - test run successful.

Eventually, the compressor will need to be serviced, but for now, we're going to try and get through the summer.