If you wish to give the user no access to Rex, you can remove their role or privileges. You would do this if the user doesn't use Rex, or they have recently left and you're still managing their reminders and records.
- Go to Admin then click User & Security
- Open the user in question
- Go into the Privileges Tab
- Click on Change Role
- In the new window, select No Role
- Click Change Role at the bottom
The user can still go into Rex, but they no have no access to any records, you can now re-assign any reminders. For more information, see: An agent has left - what should I do?