Ahh, it becomes clear. Some staff are incorrectly refusing to issue returns, as a form of punishment, despite not being authorised to do so. They know they have absolutely no evidence of any crime, and they think they can get away with enforcing a rule that they invented on some moral grounds of fighting fare evasion, where someone can be considered 'guilty' by the guard and given a punishment.
I suggest the OP writes to the TOC and hopefully disciplinary action will be taken against the guard, as he was clearly in the wrong.
Perhaps such guards should be identified so that in shops they can not be given any discount (just in case they were intending not to pay), and any other service they pay for should be charged at full price, unless they pay in advance as there is always that risk that they may be intending not to pay.
Of course, they'd be up in arms if anyone suggested
they may refuse to pay for something with no evidence. Oh no, that's different....