A personal ban is a champion that you ban not becuase they hard counter you (because they might not even be in your lane) but because of how annoying/frustrating it is to have them in the same game as you.

For me it's Yone, his mobility is bullshit, his damage is bullshit, his E is bullshit and his Ult is all kinds of bullshit, it just feels like they took a good hard look at Yasuo and said "How can we take this character and make an easier and stronger version of him" his kit is so bullshit that he doesn't even need to hit his bullshit ultimate so much so that he often uses it to just close the distance or run away...

This clip lives in my head!