Hey,
we use Zammad internal and external. This is very helpful because the customers can see the state of a ticket or see our solution.
You can restrict the ticket in a simple way: go to the admin panel > overviews and edit the view for your customers. You can restrict the overview by add a filter for creation date of a ticket or closing time. BUT: this is only for the overview. Customer can view the other tickets by use the direct URL of the ticket.
The only way to block that is in my opinion to remove the customer from the ticket.
I don‘t know if you can set the first post to internal 
But maybe another guy has better ideas 
I‘m not a pro 