I can say with all honesty that I have personally banned champions that a player has declared (only in ranked play though). I do this for one of 3 reasons 1) The champion is trash in the current meta 2) The champion is trash for that specific role 3) I don't want the enemy to have that pick (ie you are last pick and plan to play Yi, yeah good luck with that). I don't do this all the time, only when I feel that if I DON'T ban that champion we will guarantee lose. I have actually had some people stay in lobby and pick something more relevant to the team comp after that and we win. It's when people get mad and choose to punish us for making a ban choice that things get sour. This is why I never declare a pick. I can often times tell just by what people are planning to pick whether or not we will win. And most of the time people don't care about team comp they are dead set on their planned pick regardless. Riot gives me a ban to use how I see fit. And that is how I see fit to use it. I'm not being toxic or trying to piss you off, nor trying to be funny, I'm trying to do what I feel gives the team a better chance to win (yes I know you think being able to pick your one trick will win us the game, but that is a very conceited and quite frankly, blind way of looking at the game).
On a side note, I wish ban phase in ranked was similar to LCS. Being able to see the bans, and NOT have duplicate bans could add more strategy to the game and actually make us randos feel like a team. Give us time to talk strat, open up for more counters. Overall a more competitive game imo.