If you decide that you would only like to offer subscriptions to a specific product and disable one-time purchases, you can do so from that product’s page.
Go to “Products” in the left Shopify navigation and select or create the product you would like to offer as a subscription only. Scroll down to the subscription section, and select or create a subscription plan if one is not already applied.
Once the subscription plan is applied, you will see a checkbox that says "Only show this product with these purchase options".
Once you’ve added a plan to the product, you can view it, edit it, or remove it from the same section:
Once you’ve selected this option, you will no longer see the option for one-time purchases on your product pages.
Warning: Do not do this step until you’ve installed the theme code and launched subscriptions, otherwise you will disable all purchase options for the product.