To understand what to do, you need to understand the motivation of the user. Why are they doing this? If you can figure out why they're doing this, then you can figure out what the best approach is to get them to stop.
Obviously, this user has been burned before, and they are now bitter and angry. They must either have had a poor experience on a different site in the network, or they are posting from an alternative account. In any case, you are dealing with a user who is frustrated and upset.
Now... you've listed three entirely different scenarios in your question, by including these examples:
how poorly the site engine works
how hostile the company [is] towards the users
how horrible moderation is
All of these potentially involve totally different approaches, because the root cause is very different in each case.
Let's start with the first case, then.
Complaints about the site engine.
This can mean a few different things, but I'm going to assume that you mean complaints about the very basis of how the site works - in any case, far out of scope for a per-site meta.
I would point out to this user, politely, that a per-site meta is not the correct place to bring up grievances about the core system behind the site. If they can formulate their complaints into a constructive discussion, they are welcome to post it on the network-wide meta site, which would be a more applicable place for such a wide-reaching complaint.
It's worth considering that as a moderator or employee of said network, you are inherently biased - the system works for you, so it's naturally harder for you to understand why it might not work for someone else. It's often worth stepping back and considering whether they might in fact have a point, instead of just automatically assuming that since the system isn't working for them it must be a problem with the user.
Work with the user to help them turn their complaints into constructive criticism and suggestions.
Perceived hostility from the company towards the users.
If you're an employee, do not attempt to moderate this publicly. You have a conflict of interest here and you cannot act as a disinterested party. Leave this in the hands of non-employee volunteer moderators, who do not have that same conflict of interest.
If you're a moderator, gauge the post. Is it possible to edit it to be non-combative? Is it actionable?
If it's totally just a rant, you can outright delete the post. There's no need for any sort of special treatment here; if it's an unfounded rant, there's no need to keep it around.
If, however, it's a reasonable complaint, or possible to edit it into a state where it's useful feedback... it's sometimes worth it to keep around posts like that, allow for a response from the community, and let the votes fall as they will.
It's a judgement call. It's also highly dependent on the content, tone, and context of the post. It's going to be very hard to write a one-size-fits-all answer here, because every situation is simply unique.
That's where you have to know your community, and why you have moderation powers: You're trusted to make a decision. Make it. You know best in this situation.
And if you wind up making the wrong decision once in a while... that's fine. It's a per-site meta. You have a team to consult with, or other employees to ask. If something is devolving, or not as you thought it was at first, it's perfectly fine to change your mind and, say, delete the post.
Again: It's worth considering where the user is coming from. How did they wind up in a situation where they're complaining about moderation? Did we not do a good enough job of communicating our expectations? Did they simply ignore any guidance given to them and are now floundering? Is the moderation on this community different from other places on the network that they're used to?
In my experience, it's very rare for one of these complaints to result in a constructive discussion, but it has happened. You have to take into account the context of the post and of the user and make an informed decision about what you want to do. Is it worth leaving undeleted? Is it worth leaving it untouched and allowing the community to answer? Is it an abusive rant that should be deleted outright?
Also, beware of moderating posts that complain about actions that you yourself have taken - there's a natural conflict of interest.
And a few general points here...
Remember, there is a Code of Conduct. Frustrated users don't get a pass for being rude. If at any point they cross a line, you are entirely within your rights to hit them with the book - an "abusive to others" mod message covers an "angry-and-so-got-rude" situation quite well. A 24 hour suspension is also an option.
Consult with your team. Even if you personally can't work with a user for whatever reason, it's possible that one of your co-mods or co-employees can spend the time and gently guide the user.
You have been trusted to make a judgement call. You should know your community; you should be able to gauge what's best for them. It depends on a whole lotta context that's hard to spell out in a general post. The network environment, the mod workload, the activity of the site, the prior posts of the user on the network... all of these are factors that might influence your decision.
We can give you some general advice, but any situation that arises is going to have to be your call.