Banning a champ resulting in int. feeding

So I have just experienced a thing that might lead to a healthy discussion: A player in my team hovers yas mid. I'm asking if he can play something else. I have bad experience with yasuo's and besides that, otherwise we are full AD and the enemy can just stack armor. Just a 'no' comes as a response. In my turn I decide to ban Yasuo, which is, correct me if I'm wrong, not against the rules. Team mad, but okay, sure let's go with it. The guy picks Jayce, well okay. Too bad but at least I tried. When the game starts, first thing they do is go top (my lane), tell the Varus (which was my lane opponent) to come to tower, and proceed to feed him 4 kills intentionally. I think you can imagine how the game went on from that point. So as a question: Am I in my right to ban a pick? Should I be?
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