...but for the Now TV Stick there is no such option under: "Settings > System > Advanced System Settings > Device Connect"
There is a different but similar option in there, about allowing other devices (mobile apps) to connect/control the stick (Default, Permissive, Disabled), but setting that to Disabled & restarting the stick didn't remove the WiFi Direct network.
Perhaps the remote control supplied with the stick uses WiFi Direct to communicate? And is this not the case with the Now TV Boxes? So perhaps by design it isn't possible to disable it on the stick?