Never.
Have I wanted to? Absolutely.
Was there any point to being courteous with a customer who was clearly on his way out? Absolutely.
Why? ...
Your level of professionalism reflects on your business well beyond the disgruntled customers. There are a million frustrating possibilities, but you don't have to get sucked into them. Be honest and factual.
If you are a 'hot under the collar' type person, this limit will exist in all your dealings (not just with your 'bad' customers) and getting a handle on objectivity pays in spades!
You might be surprised how often a bad situation can be turned around into a really good one.
Sometimes, you just cannot fix a problem a customer is having - time to gracefully move on.
I've had numerous thank-you letters from customers who had tech problems on their end (I invested lots of time trying to help them through) and we had to part company - they were impressed by the courtesy, and there is no doubt that when they do get their technical act together they will come back.
At the very least, by being decent with those you do (and don't do) business with, you will not be contributing to any badwill.
I'm sure some of these concepts must seem pretty goofy to the popup console terrorist types, but if you want to build a business which has a better chance to endure, quality is a good angle.
Customer Care Guide
-Dino