Owner blocker. It's a method used by some, where they tell a user to get a new hobby, cos the PC is banned.
If they're doing it when you're out, disable the network at the router. If they're rebooting once left alone, but in the house/office, then you can't trust them, so they're banned. Don't look for an overly technical solution, there will always an overly technical hack for it. Disable F8, your system is inaccessible to you too. Set a password in BIOS is no good, cos the BIOS can be reset. There's always a way round these things, or you box yourself into a bad situation if the PC goes belly up.
Easier to ban the user until they learn.