Is there a quick way to undo a sandbox refresh? I just noticed the settings were set to only admins instead of all users?

Never had this request, but if there isn't a button on the sandbox management page, then you could try contacting NS support if it's still in process. If it's already finished, you are likely out of look but could likely not launch it, and refresh again w/ desired settings.

Or just enable access for any other users after it finishes...