Personally, I have a rating scale for whether I report someone who throws out "ez."
I report if:
The summoner is on the winning team (mine or theirs, I've even reported those on my friend list). This actually bothers me more when it was a hard-fought game and the fight has been going back and forth since the first minion was farmed. It's just massively disrespectful to a good opponent and an extreme demonstration of arrogance.
The summoner is 200/0 and one shots a squishy. That's just dumb, of course it's EZ, now it's just rubbing it in the opponent's face.
I do not report if:
The summoner is on the losing team. If that's the only thing they said anywhere close to negative, there's no reason to report. If they've been toxic for the whole game, that's not the reason I'm sending in the report.
The summoner pulls off a spectacular play. Come on, everyone has that moment where they do something amazing that was very risky or was knowledge of the champion (as opposed to an insurmountable gold lead). I'll let them have it, most of the time they've earned it.
Generally, I agree, calling out "ez" in all chat is just poor sportsmanship; but I try to give the benefit of the doubt to those who are using it to be a bit light-hearted (at the end of a game or when they got massively outplayed) or to those who truly feel accomplished (see above description).
Just my thoughts.