I've produced a patch that enables a numbers option to the [alphabet] shortcode which defaults to 'true', but when set to false, makes the shortcode result in displaying only letters, not numbers, i.e.
ABCDEFGHIJKLMNOPQRSTUVWXYZ
instead of
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ
I've also simplified generation of numbers and letters by usage of range() a bit as part of this patch.