There are three standard reasons for declining a flag:
flags should not be used to indicate technical inaccuracies, or an altogether wrong answer
a moderator reviewed your flag, but found no evidence to support it
flags should only be used to make moderators aware of content that requires their intervention
Ignoring the second for now, the first is used in cases where someone has flagged rather than down-voting or editing the post to correct the inaccuracies.
The third would clearly cover your second case - trolling - so flag away.
The other two - sarcasm and jokes - are (to me) less clear cut. However, if they answer the question then you shouldn't flag (see point 1 above). If they don't answer the question then flag away.
To take your example from the comments:
Q. What do I do when X happens?
The first answer:
This could be a valid answer - however it's not a useful answer as it doesn't explain anything. It's just a statement.
In this case you should really just comment and/or down-vote. However, if the poster shows no sign of coming back and making it an answer then flag it.
The second answer:
Take them out for a beer and see what happens.
This is clearly not and answer and should be flagged. If I saw a flag on that I would delete the answer.