Keep in mind that the complaint is not 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. That is 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 accordingly.
People tend to modulate their tone in kind.
Remember, just because the customer is upset, does not mean that he is wrong. It can sometimes be a challenge to wade through the emotional message and get to the basic issues. Until you find the core of the problem, you cannot 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.