How to prevent form being submit in mixins when validation failed

[Fight Ice]

I have a mixins to validate the username field and email field using ajax.
but in my ValidateField.js file, i use: 

Event.observe(this.sourceField.form,'submit',this.doSubmit.bindAsEventListener(this));

doSubmit:function(event){
        return this.state;
}

but the form still submit even the "doSubmit" return false.