If you've chosen to cancel your BoothBook subscription, log into your BoothBook account, go to Configuration (cog icon) > Account & Billing and choose Cancel Subscription at the bottom of the page.
If you cannot access your BoothBook account because you've already disconnected your subdomain (eg. booking.mybusiness.com), you'll will need to re-add this A Record in your domain hosting account and point it at the correct server IP address (further instructions of How to Re-add your A Record).
Once re-added, you'll shortly be able to access your BoothBook account again. If you're not sure of the correct server IP address, please email firstname.lastname@example.org.