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Is your actuator a brush type permanent magnet DC motor?
If so, then use a relay to switch power to the motor and when the relay is de-energized short the motor leads together.
You can do this with a SPDT relay switching one line to the motor. This is the broad concept. To do this the moving contact is connected to the motor wire. The fixed contact that is made when the relay is energized, normally open contact, goes to the power source. The other fixed contact goes to the other motor lead.