If you decide you would like to offer one or more products only as subscriptions, you will need to make this change in your Shopify admin. This lets you decide on a product-by-product basis, instead of needing to set the option at the group level.
To remove the One-time purchase option from a single product:
Open your Shopify admin
Click Products to load all of your products
Click on the product you wish to modify
Scroll down until you find the Subscriptions card
Check the option that says Only allow purchase of this product as a subscription
Save your product
Next time you load this product on your store, only subscription options will be available for this product.
Note: this is only true if you are using a storefront widget which checks to see if the one-time option is available. All of our default widgets check for this.