I got solution, i added OnClientDropDownClosed javascript function to RadComboBox.
<telerik:RadComboBox ID="cmbExpCTC" runat="server" MarkFirstMatch="true" EnableLoadOnDemand="true" OnClientDropDownClosed="OncmbExpCTCDropDownClosed" >
</telerik:RadComboBox>
function OncmbExpCTCDropDownClosed(sender, args) {
sender.clearItems();
if (args.get_domEvent().stopPropagation)
args.get_domEvent().stopPropagation();
}