I deal with people every day, and most of them are people who spend money where I work. Some of them are also vendors who take our money when we need to rent equipment we don't own. Most of these clients are polite, professional, and often times downright friendly. Other tend to be either distracted or so businesslike that they might come across as stand-offish. Most of the time, those people are in a hurry or they have many things to deal with for their job, meaning they can't joke around with us too much.
But now and then (and this genuinely boggles my mind) someone will simply fly off the handle and just swear up a storm, either at me or one of my coworkers. I don't get it!
I read stories all the time of agents and editors who have the same thing happen to them. Seriously, what is gained by losing one's temper? Absolutely nothing! In a world where people generally have to do business with each other, it makes sense to, at the very least, treat everyone else with cool, distant calm. Even when they make us mad.
That doesn't mean be a doormat, either. And don't run away from confrontation. If it turns out that choosing not to do business with someone is the best option, make that choice, but do it in a manner that keeps you above reproach. Snide emails that explain why you're not doing business with them won't help. For that matter, telling them that you'll no longer be doing business with someone may be harmful to your career goals in general. If someone's said or done something to offend, see if it can be worked out. If it can't, shrug your shoulders and move on.
Most of all, if expressing anger is necessary, keep the language CIVIL. No matter what anyone says, swearing is ALWAYS inappropriate in a business context. ALWAYS, ALWAYS, ALWAYS!!!
So that's my little tip. Behave professionally, and it'll be difficult to get blocked from future business because of something you said or did. As a writer, it means that the ONLY thing keeping you from getting published is writing quality and/or marketability.