This example shows how to make a non-deletable block. Just add data-noneditable="true"
attribute to any tag.
See more how Noneditable blocks work.
<!-- element -->
<textarea id="entry">
<h1 data-noneditable="true">...</h1>
<p data-noneditable="true">...</p>
...
</textarea>
<!-- call -->
<script>
let app = Redactor('#entry');
</script>