You can use a 'profile'.
When the machine first boots (after the jumpstart), it will load most of the manifests as services. Then it will try to act on the profiles in /var/svc/profile.
The system will come with a 'generic.xml' and a 'platform.xml'. In addition, if you create a 'site.xml', it will be used. There you can enable or disable particular services. Just copy one of the others and follow the form to enable or disable the services.
You can't disable them in a finish script directly because most of the services don't actually exist until the first boot.
The profiles are only loaded and run one time (the first boot that the file is seen). So if you use a site.xml, changing it later won't do anything at all. It'll just be ignored.
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Darren