In SLACK, it is not possible to hide groups from being selected by a user. We use groups to drive workflows and I am having to give them long, clunky names to try and discourage users from simply selecting any group (including default/unused groups and 2nd/3rd line groups are added as the assignee in the first instance. This screws up our TRIAGE workflow. This seems like an obvious fix to be able to limit the displayed groups (in the same way you can limit which forms are displayed to customers in the Zendesk "Submit A Request" form