I had exactly this problem too. Our site sells individual paintings so once one is sold, it is never going to come back into stock!
I found the solution by editing two files and changing “Sold Out!” to “SOLD!”
The first file controls individual products:
/wp-content/plugins/woocommerce-sold-out-products/templates/single-product/sold-out-flash.php
<?php echo apply_filters( ‘woocommerce_sold_out_flash’, ‘<span class=”soldout”>’.__( ‘Sold Out!’, ‘wc-sold-out-products’ ).'</span>’, $post, $product ); ?>
The second controls galleries of products and may be found here:
/wp-content/plugins/woocommerce-sold-out-products/templates/loop/sold-out-flash.php
I hope this helps!