You need to press the *stop* key that can be configured in the setting menu. As mentionned by [TankorSmash][1], pressing the stop key is always used to perform an [animation cancel][2] with skills that have non-zero cast time.


  [1]: http://gaming.stackexchange.com/questions/185728/how-to-stop-meepos-poof/185733#comment251872_185733
  [2]: http://gaming.stackexchange.com/questions/130145/how-does-animation-canceling-work-is-it-a-valid-technique/130159#130159