The TSA has no power to detain a passenger. You may not be allowed to proceed to your gate until they've determined the suspicious item is not a threat -- but you can certainly make a decision not to wait in an area designated by the TSA -- and walk away in the opposite direction.

Faced with such a situation, I'd depart the area and head back to the ticket counter if I really thought that I would miss a flight.

Actually, I have departed a screening checkpoint -- after screening had begun -- and returned to the Delta counter to be re-booked when the local "blue shirt" and I disagreed on the screening procedure at IND.

Delta was very understanding and re-booked me for a different flight and recommended / gave suggestions to enter another checkpoint in order to evade the same "blue shirt' again.

It worked -- and worked quite well.