Have you had a look at the Discourse Moderation Guide? There are lots of good tips on how to deal with different issues that may arise in your community.
Generally, the rules should be applied equally across everyone so there can’t be any disagreements about unfairness/inconsistency etc. As for the example you mentioned above, it depends on the context of the upset user. Are they frustrated about something but are posting something constructive eg. how something could be improved or are they are just being plain rude?
There are always things that users should be outright suspended for but in most cases if the user seems reasonable, I’d recommend reaching out with a PM or an official warning and try and find out what is wrong. You just have to find what works well for your community. A good place to start is the ToS and FaQ page on your community if you want some ‘policy’ to work off.