I think this answer and this GrahamHill answer are very good. Both answers make a pretty good effort to explain the reason why a "standard" solution (antivirus, antirootkit, ...) is often not enough, and both answers use, at least in my opinion, a very clear language.
It is true that nuking from orbit is not a "very accessible" option, maybe for economic reasons or because the user doesn't have a clean backup of his personal data. However, there are a lot of situations in IT (not only related to security) for which a user calls tech support, only to hear that he should buy another computer. It is not pleasant, but sometimes it is the only way. Although we are not a tech support website, I feel we have the duty to answer honestly, even if the solution is, so to say, a bit brutal.
Of course, there are also cases in which nuking from orbit may not be required. There was a recent question (that I can't find now) in which the OP explained that a friend told him he had gained access to his phone. In my opinion, nuking from orbit simply because a friend told you that he hacked your phone is slightly an exaggeration (who would want to throw $500 or even more out of the window because of a potential prank?), although it may be actually necessary.
I don't know if a canonical answer should be written, but if it was, it should include in my opinion:
the fact that a weird behavior of your PC/MAC/smartphone is not necessarily due to malware;
basic ways to understand, or at least guess, where is the problem. For example, if you use Firefox and all websites redirect to a malicious website, but in Chrome everything works fine, you could run a complete scan and uninstall Firefox, and this might be enough. Of course,it may not work, which leads me to the next point
tests and suggestions in increasing order of difficulty and "brutality", for lack of a better word, where the most brutal suggestion is clearly nuking from orbit. For example, in the case of pop-up ads, or a malicious Chrome plugin, I would point to https://support.google.com/chrome/answer/2765944, which contains a lot of useful information, and explains how to run Chrome Cleanup Tool, reset browser settings, and so on. Only if these suggestions fail, we should tell the user to nuke it from orbit;
- we should also include a basic explanation of why certain drastic measures, such as nuking from orbit, may be necessary;
possibly, the suggestion to seek advice from technical support. While this is a community of InfoSec professional (not including me), and there are really expert people here who I respect and admire, some of us might be tempted to say "nuke it" when we are not certain, and often we cannot be certain because we don't know the full story and/or because we don't have physical access to the device. An experienced technician with access to the device, and more time to ask questions to the customer, might find an alternative solution.
Just to be clear, I'm not saying that we should answer this kind of questions with "Go to tech support. Period", but I think that before making the ultimate decision to nuke it from orbit, the user should have the possibility to try for the last time.
Best-case scenario: the IT guy manages to repair the computer. Worst-case scenario: the IT guy tells the user to throw the computer away. In my limited experience, cost of repair < value of the whole computer, partly because, where I live, IT repair shops also sell computers, but YMMV. If this is the case, the user doesn't lose more money from calling tech support than he would from straight-up nuking the computer.
As I'm not a professional, I don't volunteer to write a community answer.