Unfortunately no, they can't be sued. They may be spouting out ridiculous fear mongering crap but they always start that fear mongering from a single innocuous fact. What they do from there they call "speculation" and opinions.(I put speculation in quotes because that's what they hide behind to pass off lies) Even if you can get past that you run into the first amendment, which if it can be used to protect asshats like the WBC it will certainly be used to protect Fox News.
Rest assured that Fox News has it's ass well covered in the legal loophole department. Best advice is to just stop watching it and convince others to stop watching it as well. As is said frequently around here; vote with your wallet, and in this case, vote with your TV time.