you can acces html element from server side code if you add runat="server" property to it....
then also remove an element is quite difficult.. but you can try some indirect ways...
eg...
<div id="divparent" runat="server">
<div id="divchild">
//your div content here
</div>
</div>
now to remove divchild you can try this from c#...
if(your condition is true)
{
divparent.InnerHtml = "";
}
you can try
divparent.InnerText = "";
or
divparent.Visible = false;
you can do this without setting runat="server" using javascript...