How to disable add button or Disable dialog box propagation

Question

I Have jtable and I added a button for adding record to jtable in the same page. While clicking add button new dialog box will appear to add items to jtable. In that dialog box contains a drop down and two textbox. Here I want to block the propagation of dialog box when there is no items in the dropdown list to add (I have already added a scenario where if one item added it will disappear from dropdown). how can I block dialog box propagation or how can i disable add button if there is no items present in dropdown list in the "add dialog box".

And I'm using an ajax call to fill dropdown

$.ajax({
            url: '/Program/CategoryAdded/',
            type: 'POST',
            dataType: 'json',
            success: function (data) {           

               $('#Edit-CategoryID').children('option:not(:first)').remove(); 
               $.each(data.Options, function (i, Options) {      
                $('#Edit-CategoryID').append($("<option></option>").attr("value", Options.Value).text(Options.DisplayText));
                 });
            }

and Here "#Edit-CategoryID" fills the dropdown. Here I got the count of items in dropdown using

var length = $('#Edit-CategoryID').children('option').length;
                    if(length <= 1)
                        //Here I want to write code disable add button
                    }