Downvote and leave a (polite, detailed) comment explaining what's wrong with the answer.
It would be especially helpful if you leave a correct answer to help the poor asker out (and will probably net you some upvotes, particularly if you drop the WTF answer in The Comms Room :-).
If your answer is really good you may even earn yourself a Populist badge.
There's a Meta.StackOverflow question about rate-limitations, which also links to this MSO question on accepting an answer which explains it's pretty much to give other users chance to post an answer - if you see a question with the green tick of acceptance it's less likely that they'll post another answer.
Questions like these and also questions with a known correct answer (but none of the answers are marked correct) are the reason I think Mods should be able to select Accepted Answer on questions that are blatantly abandoned (whatever criteria you want for that, user doesn't exist and Q is at least 1 mo old; User hasn't logged in for 3+ months... something ...
I remember that question.
The worst part of that particular question that you referenced was that the person who asked it refused to describe why he wanted to do what he was doing.
That's a problem, because it indicates the user has probably fallen into the XY problem trap.
There may actually be reasons why this particular solution might seem to be right, ...
All I can really say is that shit happens. If it's bad, downvote it.
from How can I list my open ports on Debian?
if "-10 votes" isn't a signpost, there's very little else we reasonably can do. Oh, and:
Adding my additional requirement (non-permanent solution) would sort of change the question itself.
Yes, yes it would. In fact, that would make your question a different question, which should be asked separately.
It is, however, a related question, so your new question should also link to the existing question, along with a clear statement that the ...
Would it be legitimate to post a "follow-up"?
Yes, if you have a problem that is similar to a crap Question, ask a new question.
If yes: How can i make sure to not get closed as duplicate?
Capitalize "I" and reference the previous Question (mention explicitly why your question is different too)
When you see an incorrect/misleading answer, the first thing to do is to downvote and preferably to add a comment. If someone has already left a comment for this, upvote the comment itself.
Beyond that, it can be tricky and may have to be handled on a case by case basis.
I think the biggest problem with this is that it seems some users tend to up-vote the ...
There's a larger question here: What to do with content from deleted users?
My own suggestion is to do two things:
Remove any accept flags that the deleted user may have placed.
Convert all of their questions and answers to CW.