Customers will click sometimes on the RETURN button during fill in process of a form

Sometimes customers will hit by accident the RETURN button during the fill in process of a form. And cause this will have the same function as the "send" button, the form will immediate be sent and they will get the "thank you message". 

Of course you can force it by using obligatorily fields (asterix). But to do this for all fields ??? 

Will there be also another way? 

Thanks