I was having same requirement. and this works for me.

$(document).ready(function() {

 $("body #s4-topheader2. ul li ul li a").attr("href","javascript:void(0);");

});

May be it helps you.

Don't forget to include JQuery in your master page.

I was having same requirement. and this works for me.

$(document).ready(function() {

     $("body #s4-topheader2. ul li ul li a").attr("href","javascript:void(0);");
});

May be it helps you.

Don't forget to include JQuery in your master page.