You could enable FileVault, which will encrypt the system and turn off the option for Safe Mode, or optionally you can try booting to the OS X Recovery HD partition and activate a firmware password, which will prevent any alternate boot conditions like Safe Mode. To do this, perform the following steps:
1. Reboot with the Command-R keys held down
2. At the OS X Tools window, choose "Firmware Password" from the Utilities menu.
3. Use this utility to set a firmware password.
When finished restart, and then system should not load into Safe Mode.
However, despite these options, you would be best off getting the keyboard fixed.