If you wish to prevent a user from logging into their account, you can unauthorize their account. To do this, go to Common>>Search and Manage Users.
From there you have the ability to search for the account by First Name, Last Name, Email and other various ways. Once you have entered in the information, click Search to see results.
Click on Manage Account and select Authorize/Unauthorize Users from the drop down menu.
You will be taken to the Edit User Accounts page where you can click the Unauthorize User button.
Once this is done, the user will not be able to log in. To reverse this, just follow the same steps and click what will now read Authorize User.