The JavaScript source in this repo contains a function: deactivateAllMembersButAdmins.
This function had the intended effect when pasted/executed in Chrome's browser console on Slack's "Manage members" page (https://computer-lab.slack.com/admin) on 12/22/2018.
The function effectively deactivates a slack workspace, a similar function could be made to reactivate the users.
Then workspace could be toggled on / off during holidays or weekends.