I would recommend using jQuery, it makes JavaScipting easier to write and read:
To disable a control using jQuery use the attr() function passing in the attribute as the first parameter and value as the second.
$("#ddlc").attr("disabled", true);
where ddlc is the id of the control (which in my case is ASP.NET and in yours probably JSF):
<asp:dropdownlist id="ddlc" runat="server" clientidmode="Static"></asp:dropdownlist>
For detailed jQuery documentation refer:
jQuery[
^]
Please let me know if this doesn't work for you and I will remove this solution.