The majority of Fiverr buyers are really nice. Some are a bit confused about the platform or about what they actually want, and some don't speak English very well, which can easily lead into a misunderstanding, but overall, they're good people, and good to work with.
Unfortunately, some of the Fiverr buyers are abusive. They might be over-demanding, and/or offensive, and/or repeatedly requesting something that you don't offer (whether it's a particular service or doing revisions until they're happy, and they're never happy), and/or threatening to ruin you, to leave a bad feedback, to report you because of whatever even though you did nothing wrong...
Sounds familiar?
It's easy to get upset or angry in such a situation. It's quite understandable that you feel frustrated. However, there are some things that you shouldn't do, even if you feel that this particular client must be the worst person in the world. The short-short version would be: don't become abusive, and don't break the rules. Let's see some examples:
1. Insult the Offensive Buyer
Yes, the buyer was abusive, but it doesn't mean that you should become rude, too. Read Fiverr's Terms of Service again if you need to, and remember that you should be professional and polite. Don't let the offensive/abusive buyer drag you to their level, and don't give them the ammo if they decide to complain to Customer Support. On top of that, if you decide to report the abusive buyer to Customer Support (you should do that), and the staff sees that you were impolite and unprofessional to the buyer, you could get in trouble, too. Remember the rules, and stick to them!
2. Name and Shame the Buyer in the Forum
It might feel good to vent in the forum, but if you decide to create a new thread in the Ranting Pot (or post in an existing one), don't name the buyer. That's against the Forum Rules, and on top of having problems with someone unreasonably difficult, you will get in trouble for breaking the rules, too.
3. Create Multiple Tickets When Asking Customer Support for Help
If a buyer is being abusive, you should definitely report them to Customer Support, politely and briefly explain the problem, send them screenshots of the conversations (it will help a lot if you're impeccably polite and highly professional in those conversations), and ask for help and advice. However, no matter how upset you are, you should be patient, and you shouldn't create multiple tickets, otherwise you will only slow down the response, and you might get blocked as a spammer.
4. Do Something Rash/Unethical/Illegal Outside of Fiverr
Depending on the information you have received from the buyer, you might be able to find them (or their website, more likely) outside of Fiverr, and it could be tempting to do something vengeful in order to make their life difficult. However, remember that you're obliged to keep all the information confidential, and if the abusive buyer reports you to Fiverr, you could get banned. Is this person really worth it?
5. Break Any Rules
Try not to break any rules. No matter how angry you are, don't scream at the buyer, don't send them malware, don't post the buyer's username all over the forum and/or Fiverr social media accounts, don't... Don't do anything that would get you in trouble with Fiverr. Try to stay calm and professional, don't answer abuse with abuse, report the abuser to Customer Support with all the proof you have (and if you stay calm and professional, the chances are that you will have plenty of proof of their unreasonable/abusive behavior), and wait for Fiverr staff to take care of the problem.
In short: don't get yourself in trouble because someone was disrespectful/abusive to you. It won't help you, and that person is certainly not worth it.