"AFK" during the zombie round.

Players purposefully avoiding their zombie mission goes under the category 'camping'. So if a player who is in the zombie team at that very moment hides and avoids attacking the enemy opponents, they are considered a camper. You are allowed to stay AFK (unless the server is full, then admins are permitted to kick those who remain AFK), but only under the condition that you do not move as a zombie whenever respawning after getting killed. Other than that, you are allowed to slap/slay and warn players, and ban them eventually as well if they keep disobeying their mission.

That is at least the kind of narrative I typically impose on players. If you want to AFK, you must remain put and you cannot move to a hiding spot as a zombie. If not, either leave the server or attack your enemy opponents.