The easiest way to handle this is to set up the Matter for Task Billing and assign a Task Set where there are no Tasks available for Fees.  To do this, go into File Maintenance Manager and choose TOOLS, Manage Codes.  Select to Show Codes for Task Sets and once the list of Task Code Sets is displayed, click on the NEW Button.  Enter a Code and Description for the Task Code Set.  Click the ADD CODE Button. Set up the Code similarly to the one shown in the screenshot. 

Save the new Code and the Task Set and then assign the Task Set to the Matter in Question.  If Users attempt to enter Fees under the Client-Matter, it will require a Task Code but there will be no valid Code available.