jQuery.Drag.prototype.location  attribute     

The location of where the element should be in the page. This takes into account the start position of the cursor on the element.

If the drag is going to be moved to an unacceptable location, you can call preventDefault in dragmove to prevent it from being moved there.

$('.mover').bind("dragmove", function(ev, drag){
  if(drag.location.top() < 100){
    ev.preventDefault()
  }
});

You can also set the location to where it should be on the page.