Looking for locking "lanyard" reel mechanism

Depending on the direction of extension/retraction you could use a mechanism for :

• (won't extend unless you permit it) a extendable dog leash
• (won't retract unless you permit it) a vacuum cleaner cable retracting mechanism.

dog leash

Basic parts of dog leash mechanism.

Figure from medium.com

• example buy from ebay

Vaccum cleaner

You can see a patent like that in https://patents.google.com/patent/US5255768A/en.

This is an example part with a complete plug assembly you can buy from espares.co.uk (you can search and find something more to your specific needs).

Dog leashes allow line to run, retract, then slide a button to lock the line in place. Dollar stores have inexpensive options.

This takes care of locking after full retraction: Include a ramp and stop with a bead (on the line), so during retraction once brought behind the stop-engagement this prevents the line from re-extending.

If you want a seat-belt function: You would need to include an inertial catch, like a centrifugal clutch, that when it spins at some pre-determined velocity, it catches the exterior and engages (in your case) the non-rotating housing. Add dampers so that when the "catches" engage, breakage is prevented.