Too harmful, aggressive, and nearly insane customers are not frequent (if you see them often, maybe the problem is not in the customers). Nevertheless, it is good to have a couple of tricks up your sleeve to help deal with them effectively.
In addition to your attempts to calm down a customer, remind him that the support team is a team of experts who solve any issue of any complexity. Often, customers are frustrated and irritated just because they cannot manage to work with your product; they do not know how to use it effectively or make a mistake. In this case, all you need to do is to support them and let them know that they are heard, and you will help them with the issue and work on minimizing the weaknesses in the future.
Of course, not everything is just that simple. To handle these cases, the team must demonstrate empathy, patience, and sympathy to the customers. It is also essential to make sure that your support team has all the tools to resolve the issue: discounts, loyalty systems, special offers, and many other things that a customer will appreciate.