Hey Niall. I am sure that you really prefer an answer from Yootheme on how/when they will be addressing your question. I am just a regular Yootheme member who likes to try and find solutions to real-life problems so I thought that I would share a solution that I worked out so that a regular user wouldn't have access to pages/articles built in Page Builder from the front-end.
It sounds like what is most important for you is to make sure that the users who should be able to create/edit only Joomla articles/posts do not have access to create/edit/delete articles with the category used for PageBuilder pages/articles. The easiest way to accomplish this is to create a category that is used for all pages/articles built in Page Builder. In the "permissions" tab for the new category, you need to make sure that only "Manager", "Administrator" and "Super User" have any permissions (all other user groups are to to "denied"). Then all of the static pages built in Page Builder have to be assigned this new category.
In order to save me some time in my testing while at the same time having full control over access, I created a new category "blog-post" and created two test articles assigned to the "blog-post" category. I created a new user group "publisher-blog" so that I had full control but this is not really necessary. I then made sure that the "publisher-blog" user group had edit access for the "blog-post" category. Then I created a user that is assigned to the "publisher-blog" user group. In my test on my live test site HERE, the publisher-blog user group can not "create" new posts/articles in the "blog-post" category since this is a test and do not want others to add blog posts to my test site. In real life, this user group would need to have "create" permission set to "allow". As you can see, this user can not edit pages built in Page Builder from the front-end. If users click on the "Page Builder" button while editing their blog post/article in Joomla backend, hopefully they will see that they need to click "Close".
You can try it out by using the following login: