Some people will potentially say, "sometimes disconnects happen, Sometimes internet connection isn't always good."
I've actually never been disconnected from a game because of internet connection and I feel that is a cop out.
The best way to prevent people from AFK'ing during games in my opinion. This will be quite harsh too, potentially worst then suspensions and bans.
Add up all the LP gained and Loss for both teams and that person who AFK'd takes that lump sum as a loss to their LP.
In a team game where we rely on people we have never met to possess mental fortitude in difficult situations and people just rage quit forcing me to lose LP should not be acceptable.