Why ban your teammate's champion in ranked?

Okay, draft, sure, blind sure, extreme troll pick, maybe (I don't even full agree with this one), but someone who just wants to play in ranked and takes a normal champion for their role, why would you ban them? I can give the benefit of the doubt to someone who didn't see it or didn't realize, insta banned or something I don't know. But there's been 3 times where I've seen someone ban another guy's champion and then just shit talk them, 'Nice champ', 'Why aren't you picking him?', just taunting them after they've banned it. I hate how that's not reportable too. Is there any valid reason to do this, or are people just being a-holes?
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