There are three ways of damping solenoids:

12.1. Damping by Ohmic resistance
A parallel resistor can be used to limit the voltage surge that occurs when the power to the solenoid is cut off. As a result, however, the drop-out time increases as does the power requirement. Both are reduced as the parallel resistance is reduced.

12.2. Damping by Varistor (voltage-dependent resistor)
A Varistor may be used to damp the voltage surge at cut-off. This causes only a slight rise in power requirement.

12.3. Damping by diode
Diodes will completely damp the cut-off surge voltage, however, the drop-out time will be greater.