how to stop form default validation?

Here I am using form alter hook to add new fields to site wide contact form. In that one is "Reset" button to clear the form values when "Reset" button is clicked.

'#attributes' => array('onclick' => 'this.form.reset(); return false;')

that is the code I write to clear the fields. Its working but I could not stop default validation of form.

Could you please help me? thanks in advance