You could spoof your IP address and block it's communication to any system you wanted. For example, say you wanted to block my computer from it's computer. You could spoof my IP and have it block me by attacking it. It then blocks my system, thinking that I am attacking it.
You can do this with name servers, too. That could really mess up a server, if you blocked communication with it's name server.
I'm pretty sure you could also block it's loopback address (127.0.0.1). If you spoofed you IP and attacked the server, and your spoofed IP was 127.0.0.1, you could block itself from itself. Ouch.
Hope this was helpful.