Banning highlighted champ intentionally on your team?

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what if it's a non priority ban that is literally just for the sake of being an asshole?

It doesn't matter.

Riot had to come down with a definitive stance on the issue; either banning an ally's pick intent is punishable or it isn't.

As there are valid reasons to ban an allied pick intent (i.e. high priority champion when enemy team has first pick) and they didn't want people to troll with pick intent, they decided that bans took priority.

Now, if the player who banned an ally's pick intent begins taunting or mocking them over it, that is a different issue.

When someone is dedicated to being an asshole, they'll also do other things that are reportable. If they don't do anything else, then they may have had a valid reason for banning that pick.