I found the solution to this permissions problem:
The editing permission can be limited to 'Edit Own' -> 'Allowed', which is the default setting in all Joomla components.
You cannot (easily) limit a user group's edit permission to a specific page, but you can limit it to a single user.
But in SPPB, you cannot set the page owner in the page settings. You have to do it in the database, by finding the user's id, and then change the value of the SPPB page's 'Created by' column.
Now, when the user logs on to the site, he/she can edit the SPPB pages that were 'created by' this user, but not any other pages.
PLEASE UPGRADE SP PAGE BUILDER TO ALLOW CHANGING OF PAGE CREATOR IN THE PUBLISHING OPTIONS, LIKE IN JOOMLA ARTICLES.