Is it a good idea to use a ban against a champ who face-rolled your team last game, or carried your whole team? Unless that player is on your team, do you take the risk that they could have queued on the enemy team, or use the ban for champions who counter you? I personally ban a champ who carried either team last game on the off chance they are in the next game. Just wondering for your opinion.
I look at my champion, my team's champions, and my opponents bans thus far to make the most educated decision as to who to ban, usually looking for someone that counters multiple champions on our team or synergize well with popular champions that are not in play yet.
Then I ban 
Generally I'd say based on the counters, but I'm guilty of putting in a Swain ban one time after getting stomped by him because if I saw him again so soon I'd tilt off the face of the earth
I'd never ban a champ that carried my last game, although I do see it sometimes.
The reason is really clear: Either the player belongs in that elo, in which case they just had a good game. If they happen to main that champ and had a good game it would have been a good ban, however they could also get cocky from how well they did the previous game and end up giving up early kills for free by going to aggressive on an opponent who isn't nearly as bad as their previous one was. In anything below diamond really, no one is actually "good". There are just people who have good games, and more often than not that comes as a result of either an awful opponent or a small bit of luck in early game (and by luck I mean a won all-in where they survive with <10hp).
If the player doesn't belong in that elo (e.g. they're a smurf) they probably know it and play someone different every game. Which means they weren't going to play that champion again anyways.
Think if you ban based on your last game -
you should be banned from banning
Not unless it's a common pick that's either OP or counters your teamcomp/laner or jungler pick. It's unlikely you'll play against them again so soon unless you're like Master or Challenger.
I ban Xerath sometimes cause he tilts me
I ban this way if the enemy singlehandedly won with that champion. I'm scared I'll encounter another player who mains him haha. Otherwise I just ban champs I find annoying.
Khmblitzkhm
