Note: It is always best to remove a User's access to the site instead of deleting them. This keeps their record in the database (and all of the records of their activity on the site), but removes their ability to log in to the site going forward.
To make changes to a user, Log in to your website as a Superuser. Search for the User you would like to update. Click their username to view their User profile.
To make a user not interactive and remove login access, hover over the Users tab, and choose "Edit User Profile." Scroll down to section F and change the drop down to "Not Interactive (no login)."
To Delete the User completely, hover over the Users tabl and choose "Delete User." You will be prompted for a confirmation before the User is deleted.