Well -- narratively -- in Turn Coat, Binder was threatening to break Dresden's ward with a thrown pebble (or Murphy's bullet). Do thaumaturgical circles follow the same rules, or can they be assumed to be "flat" and can only be disrupted by something breaking the plane of the circle? In this case, the circle is a solid piece, like the one in Harry's lab, and the interfering object is a pipe bomb. The circle's going bye-bye in a few seconds anyway... but I need to know if the thrower is going to have a chance to run from the fallout or not.
However, in game terms, I certainly wouldn't make it a contest against the caster's skill. The fragility of the thaumaturgical setup before the spell is fully cast is the big weakness of thaumaturgy in general. I was thinking of a flat difficulty of 3 to disrupt the construct, like a maneuver. Now, if someone were actively trying to prevent the circle from being broken, that would be a block...