basic casperjs conf

casper-conf.js

var casper = require('casper').create({
	verbose: true,
	logLevel: "debug",
	viewportSize: { width: 1378, height: 768 }, //for real, my screen is like everyone else's!
	pageSettings: {
		loadImages: false,
		loadPlugins: false,
		//I'm using Chrome, I SWEAR
		userAgent: "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/30.0.1599.101 Safari/537.36"
	},
	//don't load any external ressource
	onResourceRequested: function(capser, requestData, networkRequest) {
		if (requestData.url.indexOf("http://thetarget.com") !== 0) {
			networkRequest.abort();
		}
	}
});
//logging all the thinnnngs
casper.on('page.error', function (msg, trace) { this.echo( 'Error: ' + msg, 'ERROR' ); });
casper.on('remote.message', function(msg) { this.echo('Remote message: ' + msg); });

You could also subscribe to the resource.requested event and abort from there:

casper.on("resource.requested", function(res, req) {
  if (res.url.indexOf("http://thetarget.com") !== 0) {
    req.abort();
  }
});

Oh, I missed that in the docs... thanks :)