The Daily Beast runs a story on how ISIS uses sophisticated online social media to recruit disaffected people in the West to attack the countries within which they live. Most of these recruits are essentially "lone wolf" DIY terrorists who have never met someone actually affiliated with ISIS.
DHS and the FBI are said to be very concerned about such recruits. Presumably almost any Muslim could become a recruit, many have considered it, and a few have acted out.
A good immediate step would be to severely limit the immigration to the U.S. and other Western countries of persons of Muslim faith. In the event that ISIS-inspired violent actions become widespread, expect to see calls for the establishment of internment camps like those of World War II.
We need a president who will say, "Militant Islam has declared war on the U.S., a war we did not seek but cannot deny. Any who aid it forfeit their citizenship or permanent residency."
We must not let joining ISIS for a spell of bloodletting, rape and pillage become a wanderjahr or walkabout for the West's Muslim youth. Make joining it an irrevocable choice.
Instead of trying to keep people from going to Syria, we should fingerprint them, let them go and revoke their passports so they cannot return. Adding them to a database of wanted felons who will be imprisoned if caught in the U.S. would be useful since we seemingly cannot control our borders.