Samsung Galaxy Tab 10.1 — how to lock device

Hi,
I have a Samsung Tab A 10.1, I have a password set but was not aware that my son had been accessing the internet via the guest login. There is no way to prevent anyone from doing this, I have since been told by Samsung that there is nothing wrong with the device.
I have asked for a full refund as the device is, in effect 'unlockable' obviously they couldn't care less and said no.