I first try to pull it using pliers on either side of the fan if it wont move I put a small amount of degreaser on the spindle and turn it on and the heat on low, close the door and it often loosens. You can hear it when its loose before it comes off and the use pliers to remove it.
There is some that have a retaining washer after you remove the nut this can be removed with thin screwdriver normally if not again I do it by turning it on. If that fails I clean it in place and just tell the customer.
I made a tool a coupe of years ago to remove stubborn NEFF fans. It should be in some of the old posts. I dont bother trying to remove the stubborn ones now. If they dont come off, I just spray with degreaser and scrubb with a small wire brush in situ. Leave for a while, scrubb again with the small brush and they usually come up OK.