Remove all blocks from the theme category. Useful if you use the WP Block Editor but not a Full Site Editing Theme.

WP-custom_allowed_blocks.php

<?php /* Remove all blocks from the theme category */
function custom_allowed_block_types ( $allowed_block_types, $block_editor_context ) {
  $block_types = WP_Block_Type_Registry::get_instance()->get_all_registered();

  if (!empty($block_types)) {
    $allowed_block_types = array();

    foreach ($block_types as $key => $value) {
      if ($value->category != 'theme') {
        $allowed_block_types[] = $key;
      }
    }
  }

  return $allowed_block_types;
}
add_filter( 'allowed_block_types_all', 'custom_allowed_block_types', 10, 2 );