Its reportable and dishonorable.
Granted I'll say ez occasionally, usually when the enemy team spends all game spamming their lvl 7 or all chat BMing and then just throw, but regardless it's a dick move to say in the end. It is infact a reportable offense but nobody has really gotten banned for GGEZ (but rather ez + a likely combination of other toxicity) so there's no restraint from doing it anyway. Would like to see riot put their foot down a little harder on unsportsmanlike BM such as ez since it also influences some tilt into your next game and muting the entire enemy team before they end is a little obsessive.