What I would like to know:
- some google key word to search for the right mechanism,
I'd say "brake". 
But, I have no idea how to build a miniature brake. Maybe a friction device of some sort, or something poking through holes to stop rotation? I suppose it would use a solenoid. If you want to engage the brake when power is off, you'd need a spring to engage the brake and the solenoid would pull against the spring to release the brake when the servo is operating.
A stepper motor can make a pretty good brake, but they are not that cheap and they can only "brake" while powered-up.