Unfortunately, it appears that most advertizers are not stopping their support of FOX News as per the quote from the LA Times below:

While the advertising boycott has generated substantial media coverage, Fox News said it has not impacted the network’s revenues or Beck’s audience. “The advertisers referenced have all moved their spots from Beck to other programs on the network so there has been no revenue lost,” a spokeswoman said.

LA Times Beck Story

Seems like an empty boycott to me. They should be pulling their adds from the network and causing them some financial harm.