Well, to be fair, if you're going to lose a game because someone on your team is feeding, why shouldn't you win games when an enemy is feeding? If you don't take the freebies, it won't be balanced and you'll have an expected win rate of under 50%.
Win the game as fast as possible (should be easy to kill the nexus around 20minutes if someone is intentionally feeding), apologize to his allies for their bad luck, report the feeder, and move on.
For the most part, I've only seen people start feeding after a game is pretty much a lost cause, or at least his lane is. Which still isn't acceptable, but you're not exactly getting fed because he's feeding. You earned being fed, and then just got extremely fed because he decided to start feeding to try to play it off as if he intentionally sucked instead of just got outplayed. i.e. if I start out 4/0 in 7 minutes, he wasn't intentionally feeding me. He could have given me a lot more kills in 7 minutes if he was trying to. I earned a really early kill, then a second, then possibly forced a 3rd and 4th with the lead I developed. At some point it stopped being me having a lead and executing it and turned in to him intentionally feeding (when I'm now 15/0 at 15 minutes), but I still 'earned' that lead. Likewise, when I cheese the enemy jungler for first blood at his first buff then he chooses to go feed another lane - it wasn't because he came in to the game planning to feed, it's because my play made him decide to feed. Again, not acceptable, and I don't understand at all why people do this, but if you make plays that make your opponents feed, I don't see why you shouldn't take advantage of that.
Not sure if that was understandable? Basically, take advantage of it and hope they get perma-banned.