You can't use your staff powers against someone who is calling you Joseph.
...people are more likely then to, out of arrogance, continue this act as you have displayed by use of authority that you resent this action. That's why the Lead Teams and Community Team are more suited to this sort of issue as they can bring a resolution concisely without messy, unjust punishments which are more likely to provoke further disruption.
You've done the right thing by requesting people to desist this, and it is their choice to do so. If you have issues with individuals - hit me up, I'll be happy to see if they will stop in the interest of morale, however you are not entitled to warn someone for doing this - as such, players who are impertinent, rude or aggressive to the staff team (unless in violation of the TOU, or causing disruption or interference with a situation) can not have action taken against them.
Take a note of that, and I'll lock this.