$(formObject).dialog({
 create: function(event, ui) {
  $("body").css({ overflow: 'hidden' })
 },
 beforeClose: function(event, ui) {
  $("body").css({ overflow: 'inherit' })
 }
});

Or as I allude to in the comment below:

var dialogActiveClassName="dialog-active";
var dialogContainerSelector="body";

$(formObject).dialog({
 create: function(event, ui) {
   $(dialogContainerSelector).addClass(dialogActiveClassName);
 },
 beforeClose: function(event, ui) {
   $(dialogContainerSelector).removeClass(dialogActiveClassName);
 }
});

But actually to be honest, you should ensure that creating a dialog bubbles an event up to your window object where you'd be watching for said events, roughly something like this pseudo:

var dialogActiveClassName="dialog-active";
var dialogContainerSelector="body";
$(window).on("event:dialog-opened", function(){
    $(dialogContainerSelector).addClass(dialogActiveClassName);
});
$(window).on("event:dialog-closed", function(){
    $(dialogContainerSelector).removeClass(dialogActiveClassName);
});

I needed to do exactly the same thing, do it simply by adding a class to the body:

.stop-scrolling {
  height: 100%;
  overflow: hidden;
}

Add the class then remove when you want to re-enable scrolling, tested in IE, FF, Safari and Chrome.

$('body').addClass('stop-scrolling')

JS Bin reference for CSS overflow

Simply Add

$('body').css('overflow','hidden');

to your function that shows the modal.

And

$('body').css('overflow','scroll');

to your function that closes the modal.