Rule one has two parts:
a. the customer is always right
b. if that's not true, it's unlikely that this person will remain your customer.
If you need to explain to a customer that he's wrong, that everyone else has no problem, that you have tons of happy customers who were able to successfully read the instructions, that he's not smart enough or persistent enough or handsome enough to be your customer, you might be right. But if you are, part b kicks in and you've lost him.
If you find yourself litigating, debating, arguing and most of all, proving your point, you've forgotten something vital: people have a choice, and they rarely choose to do business with someone who insists that they are wrong.
By all means, fire the customers who aren't worth the time and the trouble. But understand that the moment you insist the customer is wrong, you've just started the firing process.
PS here's a great way around this problem: Make sure that the instruction manual, the website and the tech support are so clear, so patient and so generous that customers don't find themselves being wrong.