<asp:Button ID="myButton" runat="server" Text="Click Me"

            OnClientClick="return showConfirmBox(this);"

            OnClick="myButton_Click"

            CssClass="btn" />

        <script>

            function showConfirmBox(btn) {

                if (confirm('Are you sure you want to proceed?')) {

                    $(btn).hide()   // hide the button, prevent future clicks

                    return true

                }

                // Prevent the button click event from executing immediately

                return false;

            }

        </script>

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

End Sub

Protected Sub myButton_Click(sender As Object, e As EventArgs)

    ' fake 2 second delay

    System.Threading.Thread.Sleep(2000)

End Sub