1.Add CSS in your page
<style>.hidden { display: none; }</style>
2.Add this script
function ClearSession() { document.getElementById("btnHidden").click(); }
3.Add a hidden control like this one
<asp:Button ID="btnHidden" runat="server" CssClass="hidden"
OnClick="Button1_Click" ClientIDMode="Static" />
4.Add this one in your code behind
protected void Button1_Click(object sender, EventArgs e) { Session["smn"] = null; }
Now on your control you just need to call the javascript above, pretty much the same as yours:
<ClientSideEvents CloseButtonClick="ClearSession()" />