You can delete an existing Company User from within your Reseller Control Panel.
Follow the simple steps mentioned below to do so:
Login to your Reseller Control Panel. See details
In the Menu, point to Settings -> Company Users and click List. This will list the existing Company Users along with the their Role.
Click the Name link of the Company User to bring up the Details view for the Company User.
Click the Delete button and then confirm the Deletion by clicking OK.