On Craigslist, the button reads "Flag this post," but you can flag a post because you think it's bad OR you can flag because you want to nominate it for "Best of Craigslist."

After you click on the "flag" button, you get the choice, as well as a comment box to state your reasoning. That seems to work pretty well (although the choices here would be a bit different) -- just saying "Help" seems a bit vague, people might confuse it with a site help function.