Hide Add New option in Cross Reference field
I need this too, and that solution will simply make the field read-only as you suggested. The need here is to force users to use the Lookup or nothing.
I have a situation where the submitter has Create access to application A and Create access to Application B. A is linked to B with a cross-reference. I want to force them to lookup a B record which is already in the system rather than have the ability to create a new B record.
But as long as they have the Create access to B, the system will show them the Add New button. And as long as add new is there, the users will continue to click it and create redundant data in Application B.