Go to the relationship and clear the "allow creation of records via this relationship" option enabled for the portal's table occurrence.
This also removes the ability to add new records directly in the portal without using a scripted button to do so.
Right, but the only problem with that approach is that (currently) it's the same portal on both the data entry area and on the print layout.
Perhaps, then, I need to make the user enter a detail record using another (non-portal) layout and then just do as you suggested for the printed layout?
P.S. Is there a way to display all records for a table in a layout regardless of relationships? I don't see how to do it. I want to be able to show related records for a project, but also "all projects" that the user could pick from. I tried an "X" relationship, but that did not appear to work.
The best way to include a portal on a print layout is not to. This is not always possible, but the best option nearly always when such an approach is possible.
The other option is to use two table occurrences with identical relationships (except for the "allow creation..." option), for the portals on your two different layouts.