If they get 5-6 kills, then rubber band their controller, they get full exp cred like everyone else, 4500 - 5950 xp per SO game. Today, 2/3 of my games had at least one AFKer, one had two. There's topics on other [exploit] sites about 'how to afk in Spartan Ops' (found them when Googling around to see if it was a major issue when I started to notice it)
It's a pain in the ass when we spawn near them, the game takes longer (albeit this does add to the score, apparently the longer the game the higher the score, and you only need a minimum number of kills to reach the maximum score for the time you got, Gagarin is sitting at about 8.5 minutes optimal time), and it's frustrating to have to carry their asses when I and my teammates that actually put in effort do all the work for them.
Either reduce the xp output of Spartan Ops games to make it less of an incentive for them to do this crap, or have the AFK system kick them after the 3rd or 4th wave if they have zero kills (As WELL as if they stand still for 60-90 seconds).