As people, including myself are still looking at this old question, here's my answer.
At first I agreed that not being able to unpublish or remove my app's available subscriptions from the Store, as is still the case in 2020, is a problem. However, although it seems not great, in fact it's no problem at all.
The list of subscriptions isn't loaded from the Store, but from the app's "own" server. Simply preventing a subscription from being included in the list, from your own server, of current subscriptions prevents new users being able to sign up.