I would expect that anyone whose account is online and active for more than 72 hours in a row could reasonably be said to be either an AFKer or someone sharing accounts - Few people can actually stay awake that long. And I think the solution to the problem is fairly simple (in theory at least). Make it so if you are in combat with a monster for more minutes than its total hitpoints, and fail to do any damage, then they get bored and wander off - removing themselves as a viable target for long enough for the attacking player to lose the active attack and become idle. The only way to resolve that while AFK would be to use a macro program, which Jagex would be able to track down.