Hello Harlen,
What you want can be achieved by using Flash block protection.
See section 4.4.6 of the manual. Maybe look at the GT16A manual for a better description.
You just programme NVPROT with the appropriate value and that's it. ($BE)
This only provides protection from internal access.
When you connect via a BDM adapter the software will un-protect the flash so it may be then modified.
Note that this un-protecting can only be done through the BDM.
If still not clear on this perhaps you can narrow down your area of concern.