Air out of memory because photos are syncing from iMac

My Air is almost out of memory and currently almost 50gb is photos. I have never synced my iPhone or camera to my Air. Rather, the photos are coming over from the photos that I sync to my iMac. How do I 1) break this link so that it doesn't continue?; and 2) remove the photos from my Air without it also removing them from my iMac?

For reference, my iMac keeps all photos, music, movies, tv shows, and books stored on it. We then selectively sync them to various devices as needed.