LT;DR at the bottom.
In League of Legends, every possible result ends up in the victim being punished by Riot.
Scenario: You get into a game with a toxic player. This person is being toxic by either being rude or purposely hindering gameplay (going afk, dying on purpose, taking buffs when they're not supposed to or building something weird because "it's lost anyways").
You can choose to ignore them but that wont stop them from diving the enemy team or tower by themselves at lvl 5. Punishment: No fun because you're losing for dumb reasons.
You can try to surrender but 90% of the time there will be at least 2 red votes. Punishment: No fun and you're FORCED to keep playing or YOU get reported.
You can try to go afk but then you won't be able to report them and they will in fact report YOU. Punishment: Getting chat restricted/banned and the toxic person gets off free.
The only result that isn't directly negative towards the victim is /ignoring all the toxic players and then lose the match because they're still ruining it for you. Since fun > everything else this makes for a very poor option, even though it's the only one that won't get YOU in trouble.
TL;DR: There is no method of avoiding toxic players that doesn't severely punish the player. This results in players themselves becoming toxic due to them feeling powerless.