You can remove the any Clients' access at any time.
If you wish to do this without effecting any of the content under the Client:
Go into Clients and click into the Client whose access you want to revoke
Click the 'Block User' button.
If the Client has no content or you want to remove them and their content permanently, simply select the Client you want to remove and click Delete.
If at any point you wish to restore the access to the Client, granted you didn't delete their account, simply click 'Unblock User' and their access will be restored.