The Guard won't stop anyone from leaving the city to hunt Kuo-Toa (Unless I got my wires crossed, this is MND's plot), because they are very confident in their ability to protect the city.
They have the watch tower, and anyone, anything, crossing the Well will be attacked by spawn, be they stealthed, invisible, whatever, (at the very least spawn will come out to sniff around for possible prey) so the guards have a heads up if anything is coming their way.
Now, if the Peacekeepers are trying to stop people from going, that's a PC matter, but as of now, no law forbids it.