You can control if only paid subscribers or everyone on Substack can reply to a note.
When composing a new note, click on the default "Everyone can reply" and a menu of two choices will appear: Everyone or Only my paid subscribers.
Select "Only my paid subscribers" and the ability to reply to your note will be locked to subscribers with monthly, annual, or founding member plans only.
If a viewer of the note or a free subscriber tries to reply, they'll see a prompt that replies are for paid subscribers only.
Anyone who is @mentioned in the post has the ability to reply.
Tip: You can edit the reply settings after the note is published by clicking on the three dot menu and selecting "Edit".