In Craft 3 this is a cinch:
class PluginController extends Controller
{
// Disable CSRF validation for the entire controller
public $enableCsrfValidation = false;
...
See the Yii2 docs for more control over inividual actions: https://github.com/yiisoft/yii2/blob/2.1/docs/guide/security-best-practices.md#avoiding-csrfhttps://github.com/yiisoft/yii2/blob/master/docs/guide/security-best-practices.md#avoiding-csrf