Prevent unauthorized image flashing I wonder whether there is a way to prevent someone to use fastboot to flash an Android device. 

Indeed, while one must be able to turn on USB debug in order to run adb, the bootloader does not offer any protection AFAIK.
You may only make oem lock, but if anybody really need to flash something - he can do unlock  again (with removing anything from device).

And one old rule: if anybody has physical access to device - software protection will not help.