If you're really at the point where you want to tell your customer to fuck off, than you probably don't care for their business anymore. So why not offer them

If you're really at the point where you want to tell your customer to fuck off, than you probably don't care for their business anymore. So why not offer them a refund, say "we're sorry we couldn't accommodate your business needs", and end it there? Quick and to the point, emotionless, and an easy way to save time/money on a bad apple.