A highly rated show will always have advertisers. While a company like General Motors might not want the public image problem of being associated with a controversial show, there are plenty of other companies that wouldn't mind in the least (internet companies, snack food manufacturers, magazines looking for the same audience, beer and liquor companies etc.)
By lessening the pool of potential advertisers for a show, you might drive down the rates a fraction, but you will not stop companies from buying time on a highly rated show.