There will be an army of immortal AI guards standing near cave's exit? Because if you can enter cave from world map, it means that an army of players with mercs can do it too, of course if they know that guards are killable, even if heavy armed and have alot HP.
I think you can do it without guards, simply non-members or kicked members won't be able to use ladder, so they simply won't get into cave, also when you entering farm, spawn points can be placed through out whole farm, so it'll be almost impossible for PKs to camp there and wait for poor bluesuiters to appear, the same with exit grid, make it at edges of map at all directions, not only at south-west. Without preview, without ability to enter cave directly. Because you'll need to make really many guards immune to all crit effects, alot HP (or even immortal) and with super puper 1-kill always crit weapon. Or only 1 guard with all those abilites, but with infinite APs, so he alone can kill an army before they kill all innocent bluesuiters + while guard(s) are killing one evil guys, some of them could be sneakers who will use that moment to hide in cave and then troll innocent in dark corners! If that place will be really popular for newcomers, then trolling them will be really fun.