There is no doubt, people can make crop circles. It is likely that ALL of them are man made. Here's proof in a video, with links to more within it.
That being said, could there be another, "natural" reason for perhaps some of the crop circles? Geometric shapes appear all over the place in nature from, Nautilus shells to Basalt rock formations and crystalline structures.
I remember, one day after a storm I watched a "point" of wind dance across a large water puddle. It lasted several seconds as it moved across, back and forth, and in what may have been circles, it was hard to tell. I have seen twisters dance around also so, my question is, could the wind actually move in a pattern to form geometric designs in a field? I might believe that before I would believe extra-terrestrials did it.