You can remove an individual subscription for the current user (users/self) by making the REST V1 /subscription/remove call -- example:
/community-name/restapi/vc/messages/id/[id] /subscriptions/users/self/type/float/remove
There is not a specific call to remove a specific subscription for a given user (other than the user making the api call). If you could make an API call on behalf of that user (for example by using OAuth 2.0 authentication via api.lithium.com).
If this is a one-off, an admin or mod could switch user and remove the subscription via the UI.
-Doug
