The server-side ID and the client-side ID are not the same. Try something like this:
document.getElementById("<%= serverSideControl.ClientID %>").disabled = true;
If you want to see an example client-side ID, just run your web application and right-click in IE and select "view source". That will show you the HTML that the ASP.Net renders to.
Also, do NOT hardcode the client-side ID you see... the client-side ID can change depending on a number of factors. Just use the ClientID property to get the client-side ID dynamically.