want to remove them all before postback

 Hello,
i am developing a chat system in ASP.NET.
i used a ListBox which automatically populated by online user after 3 sec, i put this list in AJAX update panel and put a timer of 3 sec .
now i want all the element of list to be removed before postback. i used a client side JS function and it is working but not in rite manner, some times it remove all elements on rite time but some time not. If there is some server side method to delete all element of a listbox and re-enter them again from DB ???

Please help me about it.
 

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