Something does need to be done about afks. Its tricky to deal with though, because then people could afk on purpose to save their themselves a loss.
It is really frustrating. I proposed an idea in another post that doesn't exactly fix this, but it gives some more options at least. Ways to save yourself from losing the full lp amount if you play well.
Sorry man. The only thing I can think of is if the game is stopped short on detection and then lp is calculated based on game length or something- but I haven't thought it through there is nothing easy about that one.
I go into detail about an idea to get us in the right direction about the ranked system in this post though. Like I said, it doesn't exactly solve this specific issue, but at the very least trolls and afks would quickly goto the bottom of the barrel- which in a way solves the issue... kinda.
Fixing the Ranked Ladder System
For instance by what I suggest, if you had played well and picked off some champs, ended with a good score, you wouldn't lose the full amount of lp.