--nodefaults is an option to a group itself, not to the %packages header.  It's
kind of awful syntax, but I don't think there's really a more clear way to do
it.  So you'd have something like this:

@gnome-desktop --nodefaults

instead of:

%packages --nodefaults

hrm. that doesn't seem to work either :-/

That gets me a dialog saying that the group that I'm modifying doesn't exist.
Removing the --nodefaults again makes it work.

Created attachment 299970 [details]
kickstart

That's what I get for using a really basic test of whether package groups exist
or not.