Themods can see what IP address a person is posting from. If someone posts some hate screed, gets banned and then comes back under a different name, the mods can see that the IP address matches. That's why some folks get zotted about 30 seconds after they post, the mods can tell that someone is busy in his Mom's basement again...
Yeah, they keep doing that to me on DU, too. I'm still working on getting back on, and will eventually. In fact, I'm on now. It's when I use my work computer during lunch that they get me.