Open a file called /etc/X11/xorg.conf for editing.
In that file you will see a list of resolutions that are availiable to you. Move all the resolutions you don't want into a commented line like this:
# 1024x768 640x480 1920x1440 .... etc.
This should leave only the resolution you want in the original line. Save and exit.
Restart x or reboot if that is easier for you. Your desired resolution should now be there.
Insanity: Doing the same thing over and over and each time expecting a different result.
Democracy is 2 wolves and a lamb voting on the menu. Liberty is an armed lamb protesting the electoral outcome. A Republic negates the need for an armed protest.