I would suggest responding to that post. Ultimately, unless it's a competitor trying to gain the upper hand, there's not much you could (or should) do to remove it! And if it isn't a competitor, why is that person unhappy? If you keep customers happy -ve comments and publicity are rare. One -ve comment isn't likely to do much damage. Perhaps why it was posted should be more of a concern to the client than getting rid of it?
You cannot stop people from expressing an opinion and it's unlikely that the negative publicity will go away, UNLESS it is totally unfounded. In which case you could contact the forum concerned with the reasons why it is inaccurate