There are two possibilities:

• Bend very carefully the four metallic tongues of the corresponding potentiometer that hold the end plate of the potentiometer in question. E.g. a small screw driver could be used. Pay attention not to break off the tongues. Then the end plate and the spring can be removed. Re-install the end plate by bending back the four tongues. This can be done for both or only one of the two potentiometers of the joystick.
• Another possibilitly is to compress the two ends of the spring and cut with a suitable small cutting pliers without dismantling the potentiometer. In this case the remnant of the spring remains in the potentiometer.

Pay attention that warranty is lost if the joystick is modified !