Hmm...
For people giving up and saying "gg I'm done" you can tell them "far from it", and give them a game plan on how to win.
If they are flaming yo u cos of a bad play, you can tell them to relax, and it was a bad decision at the moment, you could probably mention the correct play as well.
If people are flaming you cos you didn't go in and trade while they did (and probably died for it), then just tell them that you though it was a bad idea to join.
If they keep flaming you and saying that they'll report you, then just say "k" and ignore them.
If they start using hate speech, then I tell that such speech isn't tolerated in Lol and his account can get banned for it.
If they are flaming others and asking people to report others, then you can tell them that continuing such behavior will get them punished if they kept going and they probably should stop.
If 2 people are flaming each other, then tell them to mute each other and focus on the game.
If they keep flaming after all that, then I just ignore them (but I don't mute them).