Package serverCmdLeaveMiniGame(%client). If Slayer is enabled you might want to check if it's a Slayer minigame and they have permission to leave before running your code.
function serverCmdLeaveMinigame(%client)
{
%mini = getMinigameFromObject(%client);
if(!isSlayerMinigame(%mini) || (%can = %mini.canLeave(%client)))
parent::serverCmdLeaveMinigame(%client);
else if(!%can)
{
messageClient(%client, '', "\c5You don't have permission to do that.");
return;
}
}