The first time it happened I was able to unlock it by spraying WD-40 and tapping it with a small hammer. The next time it got stuck nothing worked. My option was to replace the entire faucet, or find a hack.
The Flow Lock tumbler mechanism, when engaged, couples the outer green metal shell to an inner metal fitting. When the key isn't inserted. the outer shell spins freely, preventing removal.
My solution was to drill through the outer shell to the inner fitting, and then run a deck screw through the two holes. This couples the shell to the inner fitting and it came off easily. Side note: I was amused by the product video for this which claimed that a Flow Lock (also known as a "Spinsecure Faucet Lock II") is "virtually impossible to remove without the key" - I did it in two minutes with basic tools and a 5 cent deck screw.
- Use a 5/64" drill bit designed for metal work.
- Drill into the center of an indent on the outer shell. It doesn't seem to matter which indent you choose.
- Maintain steady pressure on the drill.
- Stop periodically and cool the drill bit with spray lubricant or machine oil.
Disclaimer: Stealing water is illegal. Destroying private property is illegal. This hack is intended for people who own Flow Locks but can't get them off using their key.