the login info is stored in the registry (on the PC where the CAD is installed). I cannot remember the key but it's there, you'll be able to find it.
You can schedule a task to periodically clear these registry entries, perhaps.
Gergely is absolutely correct.
And it might be enough to set the registry key to read-only by some GPO if you are running Windows AD in the background.
It's actually stored in a file in 9.X version of UCCX and NOT in the registry.
here: C:\Program Files (x86)\Cisco\Desktop\config\phonedev.cfg
We run an SCCM job that deletes it from the agent PC's every night and at every login. CAD rebuilds it every time you login to it so it has to be continually attended to.
There's also a separate file for Supervisor.