Intentionally feeding, even just one game, should be at least a week long ban.
Just played the first game of my promos to Gold V. Our Blitz takes clarity/cleanse, proceeds to run into tower over and over and over again. He ended "Give me Karthus and I stop feed" he said, which I have to admit I've never heard before. Anyway, I looked at his match history and it looks like he has never done this before which means he could get away with doing this every now and then and never get banned. Actually, most intentional feeders that I've encountered do this exact thing, never getting banned because they spread their activities out over a long time period.
I think even one game should be a week long ban because that way there would be a huge penalty for trying it out which might deter people like him. There would be false alarms of course, on people having a really bad game, but if they were allowed to submit a support ticket that might help fix it somewhat.
Before someone accuses me of being salty, let me just say right now that I am. Sorry. It's frustrating.