Silvery - 8:07 pm on Sep 7, 2010 (gmt 0)
Rating a company gives some indication of activity surrounding the business - an indicator of interest.
It's best not to add fresh content about a listing you don't want to rank well, IMHO.
I think Anallawall is right - the Dissatisfied link and report option are best to use. Google will pay more attention when a few people report the same business as being wrong or defunct.