Go to Manage Users / Edit Users and then search for the user you want to delete. Click on it and then click on "Delete User." Confirm deletion when you get the "are you sure you want to delete this user" pop up button.
If someone changes their email address during the year, you can just create a new user for them. You do not have to delete them, but you can if you want.
If someone moves away, and you no longer wish them to have access, then you should delete them. The next time they try to login, they will get a message saying "username not found."