As I understand it, our reactions are dynamic- in that they are affected by a number of variables (Memories, current situation, behavioral predisposition) but we cannot change the way we will act ourselves.
Suppose I saw my sister dangling off a cliff: My fond memories of time with her are cross-referenced with my learned benevolence, and the correct electron transfers occur in my brain, the result being my body pulling hers to safety.
There was no way I could, under those exact circumstances, decide to not save her. There is only one course of action available at any given time- just like chemicals will react A particular way under specific circumstances. You can't put sodium and chlorine together and make gold.
Or, suppose I had spent like 30 minutes thinking about a forum post. There is no way I could possibly decide to not do... whatever it is I end up doing. I don't actually know for sure what that is yet.
EDIT: Ah, see? I did it, but I could not *not* do it.