如何防止将数据输入ajax组合框控件？

 <%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

  <asp:ComboBox ID="service_list" runat="server" AutoPostBack="true" DataTextField="CL_DESCRIPTION" 
                    DataValueField="CL_ID" ondatabound="MyListDataBound"
                    DataSourceID="SqlDataSource1" CssClass="RedTypeComboBoxStyle" >
                        </asp:ComboBox >
 <%@ Register Assembly="AjaxControlToolkit" Nam