Hello Holger,

Thanks for writing. I believe a couple of customers had a problem similar to what you describe here (or at least I hope so) - the solution I suggested worked fine in their scenarios, hopefully it will be useful for you as well.

Please, follow the following forum thread of implementation details and let me know if this is helpful:

 WebMenu - Stopping the click on clientside (NA-7.3) 

http://devcenter.infragistics.com/Support/KnowledgeBaseArticle.aspx?ArticleID=8625

 
    function itemClickHandler(menuID, itemID)
    {        
        var item = igmenu_getItemById(itemID);
        var menu = igmenu_getMenuById(menuID);
        var child = item.getFirstChild();

        if (child != null)
        {
            menu.NeedPostBack = false;
            menu.CancelPostBack = true;
        }
        else
        {
            menu.NeedPostBack = true;
            menu.CancelPostBack = false;
        }

    }