Is it possible to unpublish a schedule?

No, it isn't possible to unpublish a schedule. 

What to Do

The notifications of the published Schedule would have already gone out to anyone who has these enabled. We recommend sending out an Announcement in your account letting everyone know that you published the schedule by mistake and that you will be making edits and re-publishing (that way they will know that they should stay tuned for updates).

From there, make edits to your schedule and send out a new version once it is ready. You'll also be given the option to either notify/do not notify your team of the update once it has been sent out.

Please take a look at this article on creating and publishing schedules for more details on making changes to a published schedule.

Alternatively, you have the option to Revert a schedule which will bring it back to its previous state before you hit Publish. Please note that some shifts, if they were still in edit/draft mode, may be lost.