Every customer is treated differently as per their needs. You need to be calm and quiet while dealing with the customers. They are rude due to some problem regarding the product of your company. Try to listen to their complaints and find a solution.

 You can't. People can only control themselves and when they refuse to treat employees like human beings, that becomes their problem, not yours. You can say "Sir/Madam, if you can't lower your voice/stop swearing at me/trating me like a second class human being, then I will ge the manager, and we will resolve it that way." Don't lash back. If they behave like children, treat them like children.