To remove an employee, go to Administration -> Employees in the sidebar.
You can choose to disconnect an employee and also choose to delete the employee permanent.
To disconnect, click the cog wheel that you see in the far right of the employee you wish to disconnect and click "Edit".
In the Account info you can change the status, click the arrow and click "Disconnected" and "Save".
If you wish to remove an employee permanent you must first disconnect this employee. Once you have disconnected the employee you will see him/her at "Former employees" if you scroll down at the same page.
To delete the employee, click the cog wheel you see in the far right of the employee and click "Delete".