Only account Administrators can remove user licenses. 

To remove a user license, log into the account Administrator Command Center, click the "Account" Invite tab, and then click "Manage Users" in the drop down menu.Select the user from the list and click the "Remove" button (round, red button).

Removing a user deletes them from the account, re-provisions the user license for reuse in the account, and turns off access to the mobile app.