With "Knowledge Management Advanced Plugin" installation, service now offers knowledge base subscription and knowledge article subscription. When we tested this feature in OOB instance, we observed that we cannot turn off subscription feature only for "Knowledge base". We want the feature only for "Knowledge Article" but not "Knowledge Base".
The product is working as designed so there is not an out of the box fix available. However, what we can do is submit an enhancement request on your behalf. Another thing that may be useful is the ability to turn off the email notifications
knowledge subscription email notification: https://docs.servicenow.com/csh?topicname=subscription-email-templates.html&version=latest