Xorg - how to disable some video modes?

[User Gandalf]

  Hello,

I have FreeBSD 6.0 with an xorg server installed. I followed the 
instructions in the handbook, and I could create a working xorg.conf 
file. My problem is that xorg starts with 1920x1440 at 61Hz. I would like 
to have 1024x768 at 101Hz by default. I tried to add

Modes "1024x768"

Also tried to use gtf to create a 1024x768 at 100Hz mode and add a

UseModes "1024x768_100.00"

section. But I was not able to disable 1920x1440, 1600x1200 and 
1280x1024. What I do know is that I go to the control panel of xfce each 
time and switch back to 1024x768 manually.

Thanks for your help

   Les