Registration and Payment for a Registration are two completely separate things.
There is no way to block the first step of adding to cart/registering before the registrant has or has not paid.
You also would not want to stop potential registrations that may want to send a check or pay at a later date.
However within BirdEase you can help manage the payment for registrations in a number of ways.
Add copy about 'spots not reserved until payment' on multiple pages...
Register and Sponsorship pages at the top:
Admin > Registration Items > Scroll down to the content editors per page
Payment Page at the top:
Admin > Payment Options > Payment Page Contenteditor to the right
The PDF invoice on Payment Page:
Admin > Payment Options > Invoice Options(top right tab) > Scroll down add terms/agreements etc.
The automated 'Thank you for Registering - NewRegistration Email' sent to the Registrant:
Admin > Message Center > Notifications > EditTemplate (blue button to the right)
- You can customize everything about the email template there.
- Be careful with the blue 'View My Registration' button as it allows the registrant to jump right back to edit their registration.
- Do not completely erase the 'Template Subject:' field.