making Request.Form["__EVENTTARGET"] to null

[rahuls2930]

hi,
i m using asp+c#. and i do a database call when a button is clicked using __doPostBack.
the database call succeeds and after that when i refresh the page the page again makes the database call due to the value stored in Request.Form["__EVENTTAR GET"].Since it is readonly i cant make it null
how can i solve this problem??
Please help....