ODK is open source and freely sharing our work with others is at the core of what we do so this kind of protection is unlikely to become a key focus for us. In general, we encourage organizations to share their forms and expertise so that we can all get to better outcomes together.
As you mentioned at Prevent form copy off device - #5 by Rusti, a recommended approach currently would be to issue devices that you control to data collectors so that you can lock them down.
This may strike you as overly idealistic but perhaps you could see if there is an opportunity to deliberately share the forms in a way that can help position you as an expert and expand your reach. Some examples:
- https://biodiversityforms.org/ (@mathieubossaert)
- https://www.unhcr.org/sens/mobile-technology/mobile-phone-questionnaires/
If none of these ideas match your needs, you could consider adding a way to block access to form definitions in a fork of ODK since it is available for you to extend as you wish.
That said, we do agree that there is a need for greater protection of captured data and Entities as @Stephen_K_ojwang mentioned at Prevent form copy off device - #10 by Stephen_K_ojwang and that's an aspect we're actively exploring options for.