Moderators cannot remove votes on posts, and don't know who voted for a post. Moderators can:

- Delete posts
- Convert an answer in a comment
- Convert a post in Community Wiki
- Convert a Community Wiki in a normal post (which will never become a Community Wiki anymore)

If there were more than one vote, they could be caught from the script against voting irregularities, and removed, but a single down-vote is not considered vote fraud. 

If the down-vote would have been given because the wrong code, it should be removed once the code has been fixed. Down-votes are not a punishment for the user who posted, but they are given for bad posts, or the wrong information given in the post.  
As the [privilege page][1] for voting down says:

> Use your downvotes whenever you encounter an egregiously sloppy, no-effort-expended post, or an answer that is clearly and perhaps dangerously incorrect.




  [1]: http://drupal.stackexchange.com/privileges/vote-down