Another big topic here. Not even all (or most) atheists will agree that all of our choices are conditioned. But for those that do, the analogy to a billiard table is powerful. The balls interact with each other according to the laws of physics. The position, direction of travel, and speed of each ball is determined by earlier physical states of the balls. The analogy is to the universe where every atom's current state is determined by prior physical states. Since we are just a collection of atoms, and without some sort of mystical force that acts on us or some ability that we possess that allows us to violate this principle, our decisions– simply a matter of physical brain states– are determined by prior physical states too.
But you may not have to go that far. If you're trying to argue that the theist doesn't have free will in the way they imagine they do (as opposed to arguing that all of our choices are conditioned), you can simply point out that the notion that our decisions are limited is self-evidently true. Often something as simple as whether you get to work on time is not determined by decisions you made, or even could have made.
Thanks Nelson, that was a huge help