How to disable scroll without hiding it?

If the page under the overlayer can be "fixed" at the top, when you open the overlay you can set

body { position: fixed; overflow-y:scroll }

you should still see the right scrollbar but the content is not scrollable. When you close the overlay just revert these properties with

body { position: static; overflow-y:auto }

I just proposed this way only because you wouldn't need to change any scroll event

Update

You could also do a slight improvement: if you get the document.documentElement.scrollTop property via javascript just before the layer opening, you could dynamically assign that value as top property of the body element: with this approach the page will stand in its place, no matter if you're on top or if you have already scrolled.

Css

.noscroll { position: fixed; overflow-y:scroll }

JS

$('body').css('top', -(document.documentElement.scrollTop) + 'px')
         .addClass('noscroll');

Four little additions to the accepted solution:

1. Apply 'noscroll' to html instead of to body to prevent double scroll bars in IE
2. To check if there's actually a scroll bar before adding the 'noscroll' class. Otherwise, the site will also jump pushed by the new non-scrolling scroll bar.
3. To keep any possible scrollTop so the entire page doesn't go back to the top (like Fabrizio's update, but you need to grab the value before adding the 'noscroll' class)
4. Not all browsers handle scrollTop the same way as documented at http://help.dottoro.com/ljnvjiow.php

Complete solution that seems to work for most browsers:

CSS

html.noscroll {
    position: fixed; 
    overflow-y: scroll;
    width: 100%;
}

Disable scroll

if ($(document).height() > $(window).height()) {
     var scrollTop = ($('html').scrollTop()) ? $('html').scrollTop() : $('body').scrollTop(); // Works for Chrome, Firefox, IE...
     $('html').addClass('noscroll').css('top',-scrollTop);         
}

Enable scroll

var scrollTop = parseInt($('html').css('top'));
$('html').removeClass('noscroll');
$('html,body').scrollTop(-scrollTop);

Thanks to Fabrizio and Dejan for putting me on the right track and to Brodingo for the solution to the double scroll bar