Sometimes it's our duty to "rage".

When feeders defend themselves by saying they're "just having fun," ask them if it's fun to be dead for over half the game. When feeders say that "we have a bad team comp," tell them that feeding and not doing anything useful will fail in whatever team comp. When someone is AFK for half the game and they come back to start feeding, make sure they know that being 0-0-0 in base is better than being 1-8-0, and that the exp/cs they soaked could only be useful if they didn't end the game prematurely with their feeding.

Saying these things or not won't change the outcome at that point, but next game the people you were honest with won't be thinking: "hmmm I was 1-8-0 last game and I gave away every objective... I wonder if we could've won if my jungler woulda gone tank?"