Sorry to hear of the bad experiences you've had whilst using your ENCTS pass. 

I appreciate that drivers can ask to see passes, but it sounds most unusual for you to be challenged so often for it to become a problem? I've rarely witnessed any spot checks of ENCTS passes, and I'd class myself as a heavy bus user. Regardless, that is no excuse for rude, aggressive or confrontational behaviour from staff. That needs to be challenged by the company.

If you feel that it is becoming targeted, perhaps something is being displayed to the driver on their ticket machine, when your card is scanned? Is it perhaps worth speaking to Durham County Council about this?