AFAIK sorting cannot be disabled in a datagrid.
Bit late, but that sorting functionality is disabled when the source of the datagrid is a microflow.
Not sure why you would want to disable sorting ?
But assuming you dont mind the resulting randomness, actualy any field would do, alternativly Add a filed to the object(s) sorted that is a constant and sort on that.
Possibly hide the datagrid header row through your CSS file?
Sort in a seperate microflow and restrict access to that microflow once you're modeling in the Production Security level.