Ranked Griefing
There is nothing worse than having your own team counter you, especially in ranked and during your promos! I am sure many people have had times when they have a team member who just likes to go out of their way to make the game miserable for others. There should be a major penalty for players who go out of their way to not make the game enjoyable.
A prime example is if you are in a ranked game and you let your team know who you are wanting to play. If they ban the champion you show you are wanting to play, then they should get automatically kicked out of champion select and have it treated like they dodged. Give them a time penalty and make them lose IP. Several times people will get on smurf accounts and will go into games (especially in the bronze level) and will go just be completely toxic to the entire team and for the entire game. These kind of experiences make it so that the game is no longer enjoyable and it becomes a waste of time.