You could create a time profile that starts when they get to work and ends at 5:30. Configure it in the User Tab. Set the Working Hours user rights profile to their normal configuration, and the Out of Hours to a user rights profile that uses Forced Login under Telephony/Supervisor settings. Set the user Login Idle Period to 60 seconds. At 5:31 they should log out unless they are on the phone.
Can't say I have tested it, but the logic sounds right.
Dermis and feline can be divorced by manifold methods.*
*(Disclaimer for all advise given)--'Version Dependent'