how to remove broken rear wiper?

hi, iv got an astra estate x reg 1.7 dti. the rear wiper arm is broke off. the bolt has been removed and the bit on the car has been socked with wd40, but cant get it off, i want to make sure im doing it correctly. the bit left, does it unscrew or pull off?am affraid of tring to hard an braking the window !!!