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.
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.