Locking out a user

In the Users modules under the User Administration application, open the record of the user you wish to lockout and check the Locked out checkbox, as shown here:

This will cause the user to remain active; however, they will be locked out of the system. Any attempt by the user to log in or generate actions through email will fail and all their existing user sessions will be terminated.

One important thing to understand here is that other users will still be able to open tickets on their behalf as the user is currently still active.