Hi Sven,
One of the booking confirmation modes is called “by admin manually’, it allows an admin to review each booking request and decide whether to confirm it or decline. You can read the article to learn more about it.
As another option, you may create a custom template for your properties, where you may not add the calendar or availability form, but add a third-party contact form instead. You can check this article to learn more about creating custom templates for accommodation type pages.