I think the units sight radius needs to be lower or equal to their combat - acquisition range.
Since Acquisition range basically tells them how far away a unit is before they should acquire the target.
Also, if you are placing the units into the editor, you can double click them and toggle them to guard distance which basically makes their acquisition 200 I think.
Other than that I'm not sure there's a way to do it without changing the acquisition range.
(without triggering that is)