Is there a way to mass block field updates from untrusted source rather than go field by field?
Untrusted should work, no?
But you'd have to do each field, one by one. You shouldn't need to block more than 10 or 20 fields.
Block Field Updates During List Import from Untrusted Sources - Marketo Docs - Product Documentation
Import a List of People - Marketo Docs - Product Documentation