Blocking is less of a threat than malicious clearances. Rogue but well-articulated commands to land can be easily countermanded by the controllers, but a bunch of rogue commands to go-around would throw a lot of aluminum onto missed approach vectors that aren't meant to hold so many aircraft. THEN you block the frequencies. Repeat that 4-5 times a day at 4-5 major airports and chaos ensues. What countermeasure could the FAA immediately deploy? Certainly not digital encryption on the radios? No, what they would do is what general aviation pilots fall back on in the face of massive radio or electrical failure: cell phones. The pilots closest to landing may go around, but the rest are told to call the approach or high-altitude center controller's land lines. Of course, a coordinated attack by the bad guys could easily overwhelm those phone numbers, but I'm sure DHS has a task force working on this threat right now. If not now, then as soon as they read this thread and figure out it's a credible threat.