To make a text frame non-editable you should put the frame either into a non-editable layer, such as the Background layer, or mark it as Non-selectable using the PrintUI Frame Options panel. The most common approach is to use a non-editable layer. In addition, you can prevent a user from resizing or moving a frame by locking it in InDesign.
You can turn off the user's ability to change images or to add text using run-time options. Please see the HTML Form App Run-Time Options documentation available at https://printui.com/developer.
To lock the style of a text frame but keep the content editable you change the runtime option for text frames. Please see the HTML App Run-Time Options documentation available at https://printui.com/developer
The run-time options are implemented by the site developer. As the editor is being loaded onto a web page, the developer can specify any number of run-time options. For the new HTML5 editor, they are documented in the HTML5 Editor Run-Time Options documentation.
Locking frames to prevent them from moving or changing size is done by the template designer, as is putting frames into the Background layer. Please see Tips for the Designing Templates for PrintUI documentation at https://printui.zendesk.com/hc/en-us/articles/115001055674-Tips-for-template-designers.