How to disable screen recording when Mac is locked?

Hi,

I've discovered a cool Mac's feature, that on a second thought looks like (very mild) security vulnerability. Mac allows one to take screenshots and even record video (of lock screen) even when the screen is locked. They are placed on locked (but not fully logged out) user's Desktop.

How did I discover it? My MacBook's disk became full for no apparent reason, and I had to figure out why. What seems to have happened: my son found this feature, turned on recording, played with locked MacBook, and left it running, resulting in 142GB video of mouse pointer moving over lock screen on my Desktop (it probably stopped once it filled the disk).

So now I'm looking for ways to disable this :). Any hints? This looks like security vulnerability to me - anyone without login rights can fill the disk and potentially prevent Mac from working properly, so I would rather disable it.

Michael.