The FAQ pages on Riot Games' site is somewhat lacking in information - how exactly does one get punished for poor behavior in the game, what's against the rules, and what are the consequences? Could someone summarize it for me?
It must be noted that the average player will have no interaction with this system and will not have to worry about it. But, in the interests of transparency:
For chat-based offenses
League's system, the IFS (instant feedback system) will scan a player's chat if they are reported after a game (and only if they are reported. This means you are free to talk smack amongst friends with impunity).
Certain kinds of harassing chat, such as encouraging suicide or using racial/sexual slurs, are treated more severely. These are detected with a naive keyword filter.
Generally, a problem player, once having established a history of poor sportsmanship, will lose "honor" (a conduct tracking score), lose access to end-of-season rewards, honor-based random content drops, and modes like Clash. They must then earn their way back up to become eligible for those things again.
They will also climb a punishment ladder of sorts, depending on the amount of time since their last punishment:
To start with, A player will lose the ability to fully use ingame chat for 10 games. This is not a complete mute, but they only have the ability to type a certain number of lines based on the length of the game. This is to encourage the player to focus on the game rather than what's happening in chat.
If they continue to use harassing chat, this restriction will be increased to 20 games. At this point, the player has lost all of their honor score, being reset to level 0.
If the problem still continues, the player will receive a full 14 day ban from the game.
Any offense after a 14 day ban will result in permanent loss of the account.
Effectively, for chat, League operates on a 4-strikes-and-you're-out system, with one exception; the "severe" kinds of harassing chat described earlier will escalate a player straight to the 14 day ban and loss of all honor on the first offense, with the next offense netting a permaban.
Since this is a ladder, both severity and consistency are important. Using slurs twice is equally as serious as low-key harassment over a long period of time.
For gameplay-based offenses
These are taken more seriously.
Intentional feeding or other interference
Intentionally dying to the enemy team ("intentional feeding", or "inting" for short), for instance, results in a 14 day ban the first time (with attendant loss of honor and rewards), and a permaban the next. This also applies to other game-based behavior intended to harm one's own team, such as repeatedly stealing jungle camps, giving intelligence to the enemy team (like calling out your own jungler's position or weaknesses or other information that's team-exclusive)
However, it must be noted that due to the nature of intentional feeding offenses being hard to distinguish from poor play, it is exceedingly difficult to be punished for intentional feeding, even when such behavior is blatant and unambiguous. Riot tends to treat intentional feeders with kid gloves until the amount of evidence is overwhelming. The link above shows a player intentionally feeding 18 consecutive games over 3 days with no ban issued.
Since League of Legends is server-side, the only form of cheating available to the average player is the usage of scripting tools that automate one's gameplay, like tools that dodge skillshots automatically.
These generally result in a permanent ban when detected, but proving their use is somewhat difficult. A number of paid scripting tools, as I write this, exist that are easy to find, use, and are undetectable since they don't modify the game or its files.
Two kinds of boosting must be described here.
The first is when a third party is paid to play on your account, artificially raising your stats. This is a permaban when detected, but detection is rare. Even then, it is not "boosting" that the ban is for, it is for sharing one's account. The same rule is invoked when someone tries to sell their account.
Another kind of boosting is when you pay a high-skilled player (on a new "smurf" account) to play with you in ranked duo matches. While scummy, this is not against any rules and not bannable.
Alt accounts are not against any rules, and a player who has been permanently banned on one account may create a new account and continue to play with no further consequences. Riot only bans accounts, not people (excepting very high-profile cases, usually involving streamers or pros).
Smurfing is also not against the rules. It is common for high-ranked players to make new accounts to stomp newbies, to play with friends, etc.
Stuff that doesn't exist anymore:
Tribunal (where other players judge reports). Hasn't been in the game for years, was scrapped due to a months-long backlog.
Report weighting (where some player's reports are given more power than others). All reports are given equal standing. Also, one report does exactly the same amount as if every player in a game reported someone - it submits the game for automated handling through the IFS. No more, no less.
Permanent chat bans were scrapped after Riot found out that players who lost the ability to chat began to misbehave in game (intentional feeding and the like) more often.
Warnings were removed early on in the lifetime of the IFS. Your warning is the first step on the punishment ladder, the 10 game chat restriction.
Loss of honor = No end of season rewards, no Clash, no random drops.
Bad chat: 10 game chat restriction (honor reduced) > 25 game chat restriction (honor wiped out) > 2 week ban > permaban.
Really bad chat: 2 week ban (honor wiped out) > Permaban
Intentional feeding: 2 week ban (honor wiped out) > Permaban (if caught)
Boosting (giving control of your account to a smurf): Permaban
Boosting (with another player in your games): Not against the rules
Smufing: Not against the rules
Making a new account after a permaban: Not against the rules