Well, it's really hard (if not completely impossible) to prove if someone is lying about having a real life emergency. But closer to the point, we can't prevent this from happening, really. You may not have to log out for a real-life emergency, but may just have to log out if you're on a deadline for something.
People should make any and all attempts to leave the system prior to logging out, but enforcement of this protocol would be inviable. Truly, it comes down to people having to say "OOC" in-system so that everyone knows, and a later time can be scheduled for a battle. System camping is relatively easy to see with big groups, and can even be identifiable in single people if there is a habitual problem.
I think that the best option would just to let it be, and if the admins are notified or notice something out of place, to let them deal with it.
Hype
Retired A very big thanks to Dark Oddity who put my signature pic together