Within Settings > General, set Send confirmation emails when the customer registers = Don't send before clicking [Save] at the bottom of the page:
When a new customer registers, they will see the following message:
Thank you for signing up!
We are not ready to accept bookings but we will send you an account when we are.
They cannot book until their account has been confirmed via email or by an admin.
If you want to allow a new customer through to book, on the Customers page where you can filter for unconfirmed records, you can click [Resend Confirmation] which will send the customer an email (with the subject 'Confirmation instructions') to verify their account:
Alternatively, you can [View] their record and click [Confirm email] at the top to manually verify their account:
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