Click on Settings and select Manage Users.
Select the User Type.
Select a user (or group of users) by clicking the checkbox(es) next to the name(s), then click Go To and select Deactivate.
This will bring up a confirmation warning box similar to the following:
Click Deactivate Student (or whatever User Type was selected).
Note: Deactivating a user prevents that user from logging in.
To allow access in the future, activate the user.