How exactly does Kou stop people climbing the mountain again? He lets citizens leave the city to collect new slaves, so anyone who wants to climb the mountain can just do it then like Tetsuzou. Sure there are powerful endermen at the top of the mountain but even if there were no city that problem would still exist.
Does Kou have people guarding and controlling the entire perimeter of the mountain ?
Surely it would be easier for this secret anti-kou gang to just leave the city pretending to collect slaves or sneak out the city at night then climb the mountain than to kill kou?