Champs You Ban to keep YOUR team from picking!
My main 2 are:
Vayne- people in Low Elo think they can play her but 99% of the time they cannot. Letting a teammate pick Vayne is really dangerous and there is a decent chance she will just feed and then demand kills to "carry"
Yasuo- GOD I HATE BRONZE YASUOS. You build your team around them (Malph Top, Wuk Jung, etc.) and then most of the time they just feed and do very little damage. Also they like to hold their ults for whatever goddamn reason and your teamfights are dependant on Yas deciding whether to press R or not (and a lot of the time they choose not to for god knows why...)

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