In order to revoke a co-worker's access, follow these simple steps:
Sign into the app as administrator
Click on 'Settings'
Click on 'Co-workers Panel'
4. a. You can either click on 'Revoke' on the co-worker you want to revoke access from or
b. You can select the co-workers you want to revoke access from and then click on the 'Revoke Access' icon
When you revoke a co-worker/user's access, his user license becomes free again and can be assigned to another person (after inviting him and he registers through the invitation email).