Sys.WebForms.PageRequestManager.abortPostBack Method
Stops all updates that would occur as a result of an asynchronous postback.
Sys.WebForms.PageRequestManager.getInstance().abortPostBack();
Remarks
The abortPostBack method stops the currently executing postback. To cancel a new postback, provide an event handler for the initializeRequest event and use the cancel event of the Sys.CancelEventArgs class.