In order to remove agencies on a JobCast you must do so following the instructions in this article. Once a JobCast is submitted, you can no longer remove agencies from a JobCast.
Once a JobCast is started, regardless of whether the agency is a Legacy or a Community, they will have been emailed an invitation to work on the JobCast. It is not possible to ‘unsend’ emails that contain an invitation link to work on a JobCast (or any email for that matter), so it is not possible to ‘uninvite’ agencies once a JobCast has started.
We encourage you to review the agencies receiving a JobCast prior to submitting it.