If a post contains inaccurate information it would not be ideal to mark it "Unhelpful". Instead, you should reply to the thread by posting the correct information. If there were a Helpful button your more accurate reply should attract clicks.

If I do receive an Alert indicating that a post is inaccurate (rather than misplaced or disruptive) I either post a reply as described in the previous paragraph, or contact the poster and suggest an edit, but anyone could contact any poster by PM (unless the member has opted out of the feature) to suggest an edit.