There's actually some value in keeping wrong answers around, as long as they're visibly wrong.
But comments are already a bit of a distraction - keeping worthless or outright harmful ones tacked onto a post is a luxury.
So you have a few options:
- reply, noting the error (so the author can delete it or correct it).
- flag it, noting the error (so a moderator can delete it, or the system itself if enough folks flag)
- both 1 and 2
Note that up-votes counter flags to a certain degree (at least when it comes to automatic deletion). See also: http://meta.stackoverflow.com/questions/17364/how-do-comment-voting-and-flagging-work