First down vote

Voting down is how the Ask Ubuntu community indicates which questions and answers are least useful. Use your down-votes whenever you encounter an egregiously sloppy, no-effort-expended post, or an answer that is clearly and perhaps dangerously incorrect.

You have a limited number of votes per day and answer down-votes cost you a tiny bit of reputation on top of that so use them wisely. Normally, you should focus on pushing great content to the top by up-voting: this is why down-voting should on the contrary be reserved for extreme cases. It is not meant as a substitute for communication and editing.

Instead of voting down, consider flagging the post if it is spammy or offensive, or flagging it for moderator attention if the question is duplicate or off-topic. Always leave a comment for the poster and the community or edit the post to correct it.