This example shows how to mark any block as non-deletable with data-nondeletable="true".
<div id="entry-nondeletable-block"></div>
const app = Redactor('#entry-nondeletable-block', {
content: '<p data-nondeletable="true">Protected paragraph.</p>'
});
// Or mark the current block at runtime:
app.block.setNonDeletable();