In the mind of some people relational warfare never ends. A truce can be offered and people can quit battling, but some people will try to pick a fight anyway. These people do not know how to live without conflict and they expect a fight in every relationship.
Resist the urge to return fire or you will make yourself a target. A defensive response to this unwarranted attack will only empower your opponent. The person firing this verbal assault is looking for a response from you. By not responding you will unload their weapon. Your silence will also cause the target on your back to no longer be visible to this unhappy person and eventually they will move on looking for the next willing combatant.