You can cut into the material until you encounter one of the ceramic spheres, which is what will turn the energy of the tool against itself to prevent them from going farther.
They do admit that with a drill it possible to find a route between the spheres to drill through it, but that can be mitigated against.
One way to ensure that does not happen is to have many layers of the spheres, which are offset relative to each other. Thereby, even if one layer is missed, there is another layer or two behind the first one to ensure that there are no gaps. If the structure is stochastic, it becomes a statistical problem. If the structure is strictly architected, it is a matter of quality control and precision of the manufacturing. In our initial configuration we only had two sparse layers with associated likelihood of ‘missing’ a sphere approximately 1/9 based on our limited tests.
Quote from the full paper below. Also see tables 2 and 4 in it to see the difference including metal fibres had on the resistance to drilling.