I was under the impression that free will is required to break a circle, even with a physical object. Bob didn't seem to break Harry's circle in StF when he threw the escape potion to him. Do ghouls have free will enough to break a circle?
TC also has a lot about the basics of a circle.
I shook my head. “Breaking the circle isn’t just a physical process. It’s an act of choice, of will—and these things don’t have that.”
I had to restrain myself from smacking my forehead with the heel of my hand. “Because someone summoned them from the Nevernever,” I said. “Their summoner, wherever he is, is giving them orders.”
“Could he break the circle?” Murphy asked.
“Yeah,” I said. “Easily.”
Circles of power are basic stuff, really. Practically anyone can make one if they know how to do it, and learning how to properly establish a circle is the first thing any apprentice is taught. Circles create boundaries that isolate the area inside from the magical energies of the world outside. That’s why Binder’s minions couldn’t cross the plane of the circle I’d drawn on the ground—their bodies were made up of ectoplasm, held into a solid form by magical energy. The circle cut off that energy when they tried to cross it.
As it sprang to life at my apprentice’s will, Molly’s circle did the same thing as mine—only this time the grey suits were inside it. As the energy field rose up, it cut off the grey suits from the flow of energy they needed to maintain their solid forms.
And suddenly the next best thing to forty demonic thugs collapsed into splatters of transparent gook.
Binder let out a cry as it happened, spinning around desperately, mumbling some kind of incantation under his breath—but he should have saved himself the effort. If he wanted them back, he would have to get out of the isolating field of the enormous circle first, and then he would have to start from scratch.
It kicked and struggled wildly—and then changed tactics. It rolled up to a desperate crouch, extended a single talon, and swept it around in a circle, carving a furrow into the rock. It touched the circle with its will, and I felt it when the simple magical construct sprang up and cut off the noose spell from its source of power: me. The silver cord shimmered and vanished.
Also, beings on the level of the Erlking and Titania seemed to be able to break circles with force of will in a pinch. Somewhere in SmF, Harry talks about the Archive being able to break free if there was a small flaw in the greater circle on DR.
As a general rule, I would say that circles aren't great catch all defense strategies. They're only effective in limited circumstances. That being said, it would probably be a good idea to make sure everybody is on the same page about those limits beforehand if possible. Differences in expectations and understanding can be harder to handle after the fact.