just set a grub password,
but never the less if anybody has direct access to your hardware you always have lost...
so never leave your system alone.
Cause even if you have got a grub password so nobody can change those lines
btw. the knowledgebase article said exactly the same as I did. try it and you'll the....
but if he boots with a external media, live-cd or install / rescue whatever your security is stumbled all over.
And to all the other regarding teaching someone something illegal...
I guess Coimbra is one of the high criminal minds, you'll see when he asks next time which port is ssh?
I think it will be interesting if somebody needs to know how to set the ACK Flag on a modified tcp header.