Yes you can stop all subscriptions related a product that has been discontinued.
You will need to stop selling that product on your storefront so you won't get new subscribers.
And lastly, you will need to cancel all existing subscriptions so no new orders will be created for that discontinued product.
Step 1: In Shopify, change the Product Status to Draft or Archived
On Shopify, changing a product Status to Draft or Archived mode will prevent it from appearing on your storefront.
Step 2: In Ongoing, remove the discontinued product from any Subscription Group
Once you remove the product from a subscription group, this will remove the subscription widget from appearing on the product page.
Step 3: In Ongoing, cancel all subscriptions for customers subscribed to the discontinued product
Please note Shopify will still create new orders for active subscribers even if the product they are subscribed to has been removed from a subscription group.
To ensure that all subscriptions are cancelled, you will need to manually cancel a subscription on behalf of a customer.