Just give it a custom feed with only one article titled "Your feed reader sucks" explaining why they get this. Maybe even generate the contents of this article

Just give it a custom feed with only one article titled "Your feed reader sucks" explaining why they get this. Maybe even generate the contents of this article dynamically to provide diagnostic messages. That way, badly-behaved clients can't use their broken RSS to receive actual articles until they fix it and they get to know why exactly.