Prevent Silent Mode

[sidebrnz]

It's also easy to get around by adding this to the kernel line in grub:

single init=/bin/bash

[bodhi.zazen]

I would suggest :

1. Limit physical access.

2. Short of that, re-install with full system encryption (LUKS), keep /boot on a usb key and use a strong password or a key.

I advise you re-install as you have no idea what, if anything, may have been installed (rootkits).

The other methods mentioned here will help , but not as much as the above two strategies.