To begin, go to http://admgr.liebertpub.com.  Click the Options button on the bottom right of your screen:

Next, click on the Click here to configure the various ad zones link:

Next, click the pencil icon for the property you wish to remove users from:

Next, to remove users, click the red X next to their names. You do not need to hit the save button. Your changes will be auto-saved.