If your establishment is faced with dealing with an unruly customer, try to maintain a clear mental difference between you and your role. Keep in mind that the complaint isn't made against you personally, but rather against the policy, the product, or the service the customer has received. If you make the issue a personal one, you will become emotionally involved, and that's not productive. Try to remain calm. If you continue to maintain a reasonable demeanor and a relatively quiet tone, an argumentative person will sometimes tone down to meet you.
People tend to modulate their tone in kind.
Remember just because the customer is upset, doesn't mean he is wrong. It can sometimes be a challenge to wade through the emotional message and get to the basic issues. Until you've found the core of the problem, you can't resolve it. Consider taking loud or verbally abusive customers into an office or other enclosure that offers privacy where he or she can vent without disturbing other customers or employees. Once the customer is calm, then decide what can be done about the problem.