Re: Prevent Silent Mode
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.
If it is not broken, tweak it... If you break Fedora you get to keep both pieces :p