Since it is nearly impossible to prevent abusing the system compleatly...
Why not just settle for something close, but still gives a good benefit. If it is 3v5 with 2 afks and you have 0% chance to win (save a surrender on their side), I think that it would be fair to make that a loss prevented or at least a lesser form of lp loss, for the losing team and players in a series will not receive a loss. Also the winners would not receive full LP (but could still win the series if they are in one) since winning 5v3 isn't winning by teamwork, strategy, or by sheer effort, it is a win by inability to continue equal fighting. If you are worried about teams abusing this, then make it only count for people not in premades, and those who afk (quit or close the client, not just a server caused d/c) are punished more severely, like a 12 hour ban for instance.