There should be a warning message when someone attempts to ban a champion that an ally selected
Yes, there are times when people intentionally ban a champion even if they see their ally having it selected (afraid that the enemy would pick it, etc) but I would say the vast majority of times a ban like this happens, the person banning wasn't fully paying attention and just banned their normal ban. A simple warning message saying that this champion was already selected by an ally would probably fix most of these accidents from happening. The only problem I see with this is if someone runs out of time to ban. This can be fixed by simply letting the ban go through in the event someone runs out of time on the warning message.