They have infiltrated our countries, we should infiltrate their enclaves within our countries. "Sting" operations, where we send agents purporting to be muslims into the mosques, upon hearing anything resembling calls to action or brainwashing, we arrest the manufacturers of these hideous weapons being used against us. It would at least be a start, IMHO.
Freedom of Speech means you can't do anything until somebody blows something up. And then you can only go after the one who did it. We can talk about it being a war. But as long as any legal institutions are functioning it will legally always be a legal matter. The law cannot prevent terrorist acts. That can only be done by first getting rid of the law.