I looked high and low throughout the Internet looking for the same solution to this problem with my Hirschmann 6000el.
I resolved this dilemma by taking the antenna out of the car and breaking the antenna in half.
Looking closely at the mast (after sliding out the corroded antenna) and exploded diagrams I can see that there is a metal sleeve within the plastic nub and that spun around with the nut.
I got the idea of pressing a drill bit against the side to create a channel. I looked at the length of the threads on the new antenna as a guide. Once the channel was made I took a jewellers screwdriver to bend back the locking nut until it fell out. Hallelujah!