I usually say "why the hell are you doing" this or that or "dude can you not" this or that. All of which are justifiable, such as warding their own lane to prevent themselves from being ganked or making the same mechanical mistakes all game long. I think the most common recent one would be them completely ignoring my lantern as Thresh and before you say so, I do place them in dead obvious positions such as ahead of allies when escaping or engaging rather than right on top of them
EDIT: I never turn into that guy who just says 'report for trash', what I would say is 'report for <insert reportable offenses>'. I never call anyone out without including the reason for doing so, and most if not all of the times it's basic game mechanics