It's a double edged sword leave these terrorists over there and they will regroup and indoctrinate future generations to come, or let them back in and since European courts are soft they will get a lenient sentence.
Best thing to do in my opinion build high maximum prisons in some undisclosed island, and throw them in there for good, either that or send in some drones and launch some couple hundreds of hell fire missiles upon them and scorch the earth with them, and after that salt the earth where they died simple.