Splitting Terrorist Cells

How can you tell if enough members of a terrorist cell have been captured or killed so there’s a high probability that the cell can no longer carry out an attack? A mathematical model of terrorist organizations might provide some clues. The question is what sort of mathematical model would work best.

In this hypothetical four-member cell, represented by a graph, Boromir, Celeborn, and Denethor communicate directly with each other, but Aragorn communicates only with Boromir.