If so you should be able to duplicate the product page within the theme editor → remove the quantity selector on the duplicated product page → apply the new product template to a single product.
If you are on an older Shopify theme then the process is a little more involved.
What you could do is just before the closing tag in the sections/main-product.liquid file, using the “Edit Code” option under theme actions, is add this code which will hide the quantity selector for that specific product ID:
{% if product.id == 7393176780972 %}
{% endif %}
Try that and hopefully that’ll hide it for your free product.