Zen Cart Ecommerce, making the checkout button click only once!! read description

I have a zen cart shopping card. It is a one time shopping. Recently i hired an IT specialist to integrate a Virtual Pos Terminal in the site. He did it, but there is a problem. When click on the "continue checkout" button the client get charged and an xml response being send, but if the client click twice he get charged twice. I need somebody who can make the button, which is <form tag to click only once and then to disappear or to become unclicable. To make the button through JavaScript only click once!!!!