Xorg - how to disable some video modes?

[gandalf]

>
> Referring to 
> http://www.linuxquestions.org/questions/showthread.php?t=381412
> (for example), you should be able to add a "DefaultDepth 24" line in your
> screen section, with your Modes listed as Martin said. Of course, this is
> useful only if you have more than one mode listed in your xorg.conf.

I only have one mode listed, and I have DefaultDepth. But when I start 
the X server, it starts in 1900x1440.
I can attach my xorg.conf file, if it helps.
Thank you

  Les