A voice chat system is warranted.
I've been playing Counterstrike for 15 years and I never had an issue with muting people that abuse you or just be a nuisance. Some servers even would have 32 people on a server at one time (16 each side) and the mic spammers were a pain but once you muted them it wasn't an issue.
LoL only has 4 other people in chat.. Once someone gets muted by their teammates, they quickly realise that abusing any player while your muted isnt a great idea. When they then want that player to help them with an objective and they get ignored they will soon realise the error of their ways..they will think better about doing it again in the next game..
Seriously this would reduce a shit-ton of abuse from players.
There are 50 or so sites that do anonymous team chat groups where you can just give people a web-link and they can join your room and chat during the game in your web browser... This should be very simple and easy to write this into the game or chat client. When you enter the game lobby, it auto creates a chat room for your team, and has button you click on ie. [Click here to join your unique team chat] and then it loads up in the background, has a dropdown someone on the HUD where you click on them to mute/report.. (see World of tanks as something to take ideas from)
So simple..