If that firewall app is making those methods return anything other than null or a truly unique ID, then they are not a compatible android device, and someone using such a device is not in any position to complain when apps behave incorrectly.
It's worth mentioning that it's not required for the return value of those methods to be the IMEI, but it is required to be either unique or null.
I can see why you might want to permanently ban them, though. Blaming you for a bug in their software.