Two suggestions:

1. Add a clause to your guidelines making clear that no further correspondance will be entered into in the event of a rejection (if there isn't one already). You can state you appreciate this may be frustrating, but that you simply don't have time to get into specifics.

2. If you do respond a second time, simply point to the aforementioned clause, and state: "Your writing did not meet the required standards. Per clause X in my submission guidelines, I will not be entering into any further correspondance on this subject." Factual but firm. If they come back after that, I'd simply ignore them.

I actually don't think you have any obligation to be kind. Publications such as yours massively benefit the writers rather than the editor, and as such you are doing us all a favour by even looking at our articles. That ought to be the attitude of anyone submitting work.