Security is like an onion, it has many many layers.
(and sometimes we end up crying)
For preventing someone to boot your linux (or not) partition it is best to protect your grub menu.
There are two ways:
1st : To use a global password for grub
2nd: To use different passwords for every menu title.
I prefer the second method cause is a more security solution.
We need to create the encrypted password, from our terminal we type:
$ grub-md5-crypt Password: Retype password: $1$nNyIl/$2rdkv9UCclYQu1Hb0hxiQ/
For our example i typed: test
And finally we add the bellow line in /boot/grub/menu.lst
password --md5 $1$nNyIl/$2rdkv9UCclYQu1Hb0hxiQ/
title Arch Linux [vmlinuz26]
kernel /boot/vmlinuz26 root=/dev/sda1 ro
password –md5 $1$nNyIl/$2rdkv9UCclYQu1Hb0hxiQ/