According to the State Department, the following acts can result in loss of citizenship:
1. Obtaining naturalization in a foreign state;
2. Taking an oath, affirmation or other formal declaration to a foreign state or its political subdivisions;
3. Entering or serving in the armed forces of a foreign state engaged in hostilities against the U.S. or serving as a commissioned or non-commissioned officer in the armed forces of a foreign state;
4. Accepting employment with a foreign government if (a) one has the nationality of that foreign state or (b) a declaration of allegiance is required in accepting the position;
http://hamilton.usconsulate.gov/loss_of_citizenship.html
All of those listed (there are more) would seem to apply to at least some of those going to wage jihad with ISIS. IOW, we just need to enforce existing law.
Where John Walker Lindh got his break was by virtue of the fact that he was already fighting with the taliban when we arrived. At least that was part of what his lawyers argued.
Personally I would have just eliminated him there and kept my mouth shut.
US passport applicants who admit to having committed a "potentially expatriating act" have to complete and sign a supplemental affidavit. If they check the box that says "I did not intend to relinquish my citizenship" (which nearly everyone does), they're home free. They get a new passport and the affidavit goes into a Raiders-of-the-Lost-Ark warehouse.