As the Institution Security Administrator, you are able to disable and remove users for your Institution. To disable a user, click on the “add/edit users” button on the left-hand navigation bar. From that page, you can disable a user by removing the checks in the various role boxes and clicking "save". If you disable a user in this way, you can always restore that user again by rechecking the boxes. However, you can permanently delete a user by clicking the "delete user" button.
See Also:
How do I set up other users in my Institution?
What are my access rights and roles as the Institutional Security Administrator?