Below is an example, please change role to username in your code, and also you break the loop once menu item is found.
protected void Page_Load(object sender, EventArgs e)
{
if (!Roles.IsUserInRole("Admin"))
{
MenuItemCollection menuItems = mTopMenu.Items;
MenuItem adminItem = new MenuItem();
foreach (MenuItem menuItem in menuItems)
{
if (menuItem.Text == "Roles")
adminItem = menuItem;
}
menuItems.Remove(adminItem);
}
}