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You don't have to delete lyrics, just comment.
E.g.

{block name='product_reference'}
          {if isset($product.reference_to_display) && $product.reference_to_display neq ''}
            <div class="product-reference">
              <label class="label">{l s='Reference' d='Shop.Theme.Catalog'} </label>
              <span itemprop="sku">{$product.reference_to_display}</span>
            </div>
          {/if}
          {/block}

to:

{*block name='product_reference'}
          {if isset($product.reference_to_display) && $product.reference_to_display neq ''}
            <div class="product-reference">
              <label class="label">{l s='Reference' d='Shop.Theme.Catalog'} </label>
              <span itemprop="sku">{$product.reference_to_display}</span>
            </div>
          {/if}
          {/block*}

An asterisk is added to the blocks.