Unfortunately many people do not know how to complain. Many simply lack the communications skills to effectively state their issue, so resort to ad hominem attacks. You may also find that if they lack these interpersonal skills, they also lack many other skills required to form opinions by critical thinking, make effective decisions, operate relatively simple devices and read the instructions. In short thick people buy junk that is not suitable for the job they want it for, they do not have the savvy to realise this, they do not read the instructions and are unable to make it do what they want because the device cannot read their mind (not that there is much to read) and it might well be the wrong tool for the job anyway.
Deflecting their inadequacies onto someone that they perceive is in a position lower than them, who is not able to defend themselves gives them a feeling of power. They have been bullied so become bullies.
You have to develop the skills to defeat their simple little ideas.
Firstly, know everything about your products, try all of them out, know what they can and cannot do, what they should and should not be used for etc. Keep calm and defeat rudeness with, good manners, assertiveness (not anger or aggression, look up assertiveness) and knowledge. Be a figure of authority. It does not come with age, it comes with training (self or trainer sourced) and experience ( a few weeks of that will suffice).
The first thing you do is listen, then the first thing you say is "I understand how you must feel" even if you don´t know or care. Your complain is a serious matter, please let me deal with it. Take ownership, and control the situation. Please take a seat while I investigate. Then ask whatever questions of them you need to get that job done.
It is very very simple, once you have learnt to deal with your own emotions. Set them aside and do not take it personally. Be the bigger person. I have been doing this for years, and I never have anyone scream at me after I have spoken for the first time. Why? Because I always say I am sorry to hear that you have had a problem I take you seriously and I WILL deal with this for you. Passive voice always. Never say I am sorry in active voice! Then they see you as their champion, defending them against the faceless machine, not a part of it.
Always be honest. If it is not a quick fix tell them and keep them updated. Never say Mr so and so is not here or any other BS. Just say this might take a little while to sort out get contact details and offer to call in an hour, next day or by next Thursday, and keep that promise.
If it turns out (and it usually does ) that they have caused their own misfortune, do not tell them that. Say the product is designed to be used in this way for this purpose, is that what you need? If no sell them something else. If the product is faulty change it. There are a limited number of things that people complain about in any given service arena, learn what they are and have pre-arranged scripts for them all that are second nature and always work.
Keep It Simple!