AJAX :: How To Close Calendarextender Updatepanel While Javascript Popup

in my aspx page,

1. am using calendarextender bind with text box

2. button click event call th SQL stored procedure to save the form values

3. after save and get the output value from the SQL SP ( its boolean value)

4. if true i have to show the javascript alert or else nothing have to alert, so this is the code

if (CheckandBooked())
{
ScriptManager.RegisterClientScriptBlock(btnBook, typeof(string), "alertscript", "alert('Selected time period clashes with existing booking');", true);
}
bug: this is working good but while showing the alert time i can see the calender( it look like design view) the design is adjusted how to avoid this