mepal-
Sorry, I don’t know how to disable a Dell’s “fn” key.
This is the type of thing that is manufacturer-specific and may even be model-specific.
A couple of suggestions:
1) Google “disable Fn key Dell laptop”. I tried this and a few links came up with suggestions. The very first link looked promising.
2) Call Dell. Their CSR’s are generally helpful, knowledgeable, and understandable.