A very simple Anti Withers plugin, just add on the server and it will work right away with nothing to config with:

• This will blocks withers from totally spawn on your server.
  
• This can be used for survival servers to block players from annoying other players or lag the server itself. (If so, recommended to add some other way to get nether stars for players. A shop with some currency or a trade. Or not, if you don't want nether stars at all.)

Good for simple SMPs that require such plugins to not overload server.