eBooks with Textboxes

Is there a way to enable or disable textboxes with eBooks that have textboxes for entering answers to study questions?

Not to my knowledge.

I have an eBook that I've "evidently" previously answered some questions in "textboxes" but not the textboxes seem to be disabled, as are all the text boxes for all the questions in the studyguide...

This could be a bug in the book production - please share the title of the book and a screenshot or two.

The previous answers are all stored in Notes (the facet filter type:Responses might help you to localize), so you should be able to change and even delete them from the Notes tool, if the book doesn't work anymore.

Knowing God Bible Study

This book as of now does not contain any user-fillable fields I could see. However, in the white space for answers, there is white text on white background, even invisible when selected.

So I can see there is "something" in the space. E.g. for Session 3 Question 6 - I can select it and copy it, it's the text you showed:

It is important to realize that everything the Spirit does in this passage—testify, convict, guide into truth, and bring glory—all focuses on Jesus Christ. Just as the Spirit testifies about Jesus and glorifies him, we must follow the Spirit’s example.

I think this is part of the resource, I don't think this would be your private filling of this. The preface contains a section on the structure of the book, and there they say: 

Throughout the guide are study-note sidebars that may be useful for group leaders or individuals. These notes do not give the answers, but they do provide additional background information on certain questions and can challenge participants to think deeper or differently about the content.

I think it's those sidebars we are seeing here - the automated converter obviously didn't treat them right, as they should be visible (in boxes, of so possible) but not in the answer fields.