Stop Background from scrolling when Pop Up is open

If you go to my the homepage at http://www.setteradvertising.com and click Contact a small pop up will open with a form. While this is up, you can scroll and the background page will move up and down. Is there a way to freeze this so that doesn't happen. So when the pop up opens you can only scroll the pop up area if needed (for example on mobile). I have noticed that on mobile devices, the background scrolling sometimes interferes with scrolling through the box. Does that make sense?