There currently isn't a way to just modify this via the machine groups, but here's how I think you could get what you're looking to do:
If a scheduled deployment is already set up for a machine - you'd need to delete the scheduled deployment job (via Manage > Scheduled tasks). If it's indeed a scheduled deployment - those are scheduled on the target system, so you can just delete the deployment job from those specific systems that you no longer want to include in the deployment. If it's a scheduled scan with autodeploy you'll need to delete the scheduled job listed under the console system and then set up your deployment again, excluding any machines you truly don't want in the deployment.
I hope all that makes sense to you - let me know if you have any questions.
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