Small form factor over voltage protection circuit for 5V power supply
You could add a 5V LDO regulator to the 5V circuit. Something like the LD2981, however it will probably appear to work and go into overtemperature protection if a 12V adapter is plugged in and there is substantial current flowing. 

Perhaps your circuit could detect the application of overvoltage and put itself into a low current state for the duration of the overvoltage. Something like a voltage divider and ADC or comparator. That will also indicate to the user that they made a mistake and should change adapters. 

The LDO will prevent even a momentary application of overvoltage, and your circuit has a bit of time to react before the LDO heats up too much.