Well they werent technically holding the game hostage were they? No one forced you to leave, you left because someone ban your champ you wanted to play. Now dont get me wrong that kind of a dick move, but at the same time thats not reportable. If they were acting how you claim they were, you leaving just made them get away with it. You should have just played a different champion (this game has more than 130 champion, there is no way you only know how to play one champion) and then report them after the game.
As for a reporting option in Champ Select. I believe it would be abused. Especially in Ranked matches, people would use it as a free dodge if they see that their team isnt as strong as the enemies. Or like in the case of yours People using it to get a free dodge because their champion gets ban (intentional or not). Just think how long getting into a game would be if something like this is implemented!
Case in point the new champion Zoe coming out in a couple of weeks. Im sure a lot of people will be queuing up for Mid just to try and play her but im sure most people will ban her right on the spot, not only because she is new, but also because they dont want people messing up their games because they want to try out a new champion.
How can we fix this problem without also making it easy to abuse? Thats the main deal right now!