Hello,
You can find code for back button in this file:
components/com_booking/views/reservation/tmpl/form_customer.php
Button Back (Zurück), should return back to last page, if it is possible. If not, it will redirect to detail page when customer book only one object. If more, he will be redirected to the list of objects.
This is not correct for you? How it should work?
Maybe we can add there settings to choose if you want to use actual functionality, or have javascript back functionality or hide that button? I can add it to our TODO list. But it will be probably released only for Book it! 2.