I'm not certain I understand the debate here.
Certainly, you are allowed to ascribe epithets to characters you dislike, like "Bowie, the traitor", for example.
It would be bad form to RP that you ban someone from a realm if you cannot game mechanically ban them, but really it's a question of not using confusing terms. You can say "If you set foot in my realm, I will make certain you will pay for it, because I think you are a traitor."
A borderline case would be to publically declare "I will have you executed or deported" when you, in fact, cannot do so. Of course, it is always possible to roleplay that the execution is delayed or did not happen for whatever reason, but it's best to only say it when the option actually exists.
I don't think you should change your opinion of other characters based on the ban game mechanics. Game mechanics trumps RP in what happens, not in what you think.