ul.AspNet-Menu ul {
    visibility: hidden;
}

ul.AspNet-Menu li {
    position: relative;
}

    /* Add more rules here if your menus have more than three (3) tiers */
    ul.AspNet-Menu li:hover ul ul,
    ul.AspNet-Menu li.AspNet-Menu-Hover ul ul {
        visibility: hidden;
    }

    /* Add more rules here if your menus have more than three (3) tiers */
    ul.AspNet-Menu li:hover ul,
    ul.AspNet-Menu li li:hover ul,
    ul.AspNet-Menu li li li:hover ul,
    ul.AspNet-Menu li.AspNet-Menu-Hover ul,
    ul.AspNet-Menu li li.AspNet-Menu-Hover ul,
    ul.AspNet-Menu li li li.AspNet-Menu-Hover ul {
        visibility: visible;
    }