There is an option to put a BIOS password in place, so the PC won't even boot up without the password.

There should be a message when you boot up which will say, for example "Press F11 to enter BIOS setup". (It varies from one PC to another - you may have to refer to the documentation for your PC, or contact the retailer you purchased it from).

The password is usually in the Startup options, but don't change anything else unless you are really sure what you are doing!

I actually use this option during exam time to disable my kid's PC's without having to physically remove them from the room, to make sure they have one less distraction when they are supposed to be studying!