Stop submitting form after the save event

Hello there. I have done this:

var editor = CKEDITOR.instances['post'];

editor.on('save', function(event) {
    alert( 'Saving...' );        // true
    //event.stop();
});

But after that the form is submited and the entire page it's reloaded (as always when submitting a form). I want to stop the event that makes the form to be submited, like when you do it normally like this:

event.preventDefault ? event.preventDefault() : event.returnValue = false;

How can achive that? Thanks.