Clearly you have to do something about leavers, AFK and people who are being jerks. But don't do ObamaCare. Don't trade one problem for another.

Break it down into parts, and fix the parts without creating a mess.

Some suggestions:
1. Vote to kick: if all the players on a team vote to kick one player, do it. Nothing makes me more sick than an AFK bot getting IP
2. Allow a sub to come on the field. Bring him in with the exact average Gold and EXP as everyone who is playing.
3. Never ban someone, that is counter to your mission. Instead fine them IP. Hit them where it hurts.
4. Reward people who get screwed with an AFK/Leaver. Just give them a bonus IP share from the IP you took from the leaver.

Think it over

For all those of you who are asking, "Well what do I do if I HAVE/MUST leave?" Just know that you are GOING to get a leave regardless of the reason. The fault lies with you, and only with you. What you should start doing is looking at the time and deciding for yourself, "Do I have at least an hour to spare before dinner/chores/going out/etc.?" If the answer to that is no, then DO NOT PLAY. Simple as that. Otherwise, expect to get a leave point.

Also, this voting concept will just completely be abused. Newer players will be voted out constantly because they do not know how to play, because they make a few mistakes, or because the other members just don't like them. People would be getting voted out for the dumbest reasons. It's just not a good idea. This also ties in with the reporting system. I don't blame Riot for not doing anything with reports, if they don't, because honestly how do you even prove that the report is legit? You can't. People will report others just because of how they do in a game, even if they do what they can. Some players are indeed that spiteful.

As I've previously suggested, a system that replaces leavers and gives them the average gold and exp gained by their team mates would be ideal. However, it is also true that cases where the player comes in late in the game and just gets the win/loss will occur.

SUGGESTION:
However, you could also just input a system that, in the case of a game where there is a leaver, NO win/loss is counted and NO IP/XP is gained by either team. Also, surrendering would become possible whenever, at any time, there is a leaver. Knowing most players they do not like getting a loss on their record because of a leaver, nor do they like the fact that the other team is getting a win and increased IP/XP because they took advantage of a player being absent. At this point, once there is a leaver, the players could simply surrender without worrying about getting a loss and the other team getting a win. Also, since surrendering would be possible at the moment of a leaver, neither team would have to waste any more time then needed and could simply start another game as soon as possible.

Unfortunately that wouldn't work either. Think about it, then people would just leave if they saw they were losing. I notice multiple things...the "surrender" option makes ppl wussies. If there's even the slightest hint they aren't going to win, guarantee 1 person will start a surrender marathon and complain because no one will surrender. Many seem to tie wins to kills, but it's really about the turrets, you can be 20 kills behind and still win, though it may be tough, and you may lose anyway, ppl give up way too easy.

I never surrender. Ever. And boy does that piss some of my teammates off, guarantee a vote would be abused, just as the reports are, but those same teammates are completely happy when the game turns around and we win anyway. And yes, if you don't have an hour to spend on this game, don't start a game, if you're level 30 and think it's going to be over in 20 minutes, you're an idiot.

I made a thread for the suggestion and explained it more thoroughly in the feedback forum. It addresses your thought.

thanks u jebus

It is a bad implementation, since there can be royal motives for that one can leave the match. For example, there are motives of which it does not reach the time to finish the match; or that a team mate is AFK or does not play correctly.
Sincerely not as solving it but the ban I do not believe that it is the way, I till now never left an match, but I have friends who because of not reaching him the time, they had to leave the match and they get banned.
sorry for my english ;D