How do I tell this guy to go away without making it awkward?

There is this guy from a couple of my cles that has been IMing me. At first I liked him being flirty with me and it made me feel good. But then it became CLEAR that he was just into hooking up with me and said some things that most girls would slap a guy over. When he would say those really dirty things I would just stop talking to him, but he doesn't get that message I guess. So how do I tell him to stop without seeing him in my cles really awkward or worse, have him retaliate?