DropDownExtender allowing input in the textbox component

Hi,

I am fairly new to ajax and trying to use the DropDownExtender to display a set of dynamically created options.  I have this working so that i can choose from the options with the select box, but if you click on the text box you can also edit the contents.  This is not really desired as the idea of the drop down is to use preset data.

My code is:

<asp:TextBox ID="xx1" runat="server" CssClass="textbox"></asp:TextBox><ajaxToolkit:DropDownExtender ID="COAElement1DDE" runat="server"
    DropArrowImageUrl="~/img/dropDownArrow.png"
    DropArrowBackColor="#dddddd" HighlightBackColor="#e9e9e9"
    HighlightBorderColor="#dadada"DynamicServicePath=""
    Enabled="True" TargetControlID="xx1" DropDownControlID="xx1Options"></ajaxToolkit:DropDownExtender><asp:Panel ID="xx1Options" runat="server"></asp:Panel>

Any ideas how I can prevent this unwanted editing?

Thanks!