Aww...why be so mean. We can just sell them as sex slaves, like Boko Haram.
My first inclination is that when some terrorist is nabbed, put them in solitary with no contact. Then via the rumor mill, put out that they were executed and their body was put in a cesspit with a dead pig.
Since it is just a rumor, they can deny it. But it would frazzle the other Arabs. And if pressured too hard, produce the living terrorist.
Then later, rinse and repeat with the next terrorist.