[user104658]

It is water off a duck's back but that's no reason to excuse or ignore it. It's also not good business. You might think you're retaining customers by not putting them in their place, but you have to remember that other (good, respectful) customers are watching and listening even when you don't know it, and THOSE are the customers you'll end up losing when you let arsehole customers go unchecked. They'll get a bad feeling about the place and be less likely to return. Best thing to do is respectfully, but firmly, remind them that they don't have the right to be abusive and if they don't comply then get rid. Funnily enough, the vast majority of people back right down pretty quickly.