Sometimes you need direct access to sockets in java. For those times I wrote a small sample. Actually I wrote it in the third semester of my degree (for evaluating java network / swing capabilities). It's a pretty nice little application and the source is extremely simple so give it a whirl if you want a decent java based socket application.
It supports client server model and a server detection system :) . You can change your username and kick unwanted users. Adding a banning system based on user IP would not be that hard since all data b/w clients is controlled (propogated) by the server.
Link : http://code.google.com/p/multiplayergameengine/downloads/list