How to make the key flexfield item cannot be changed in custom form in R12.1.3

Dear Oracle form experts,

Currently, we are facing an issue, i created a custom form, it have some key flexfield item, the flexfield item have some segments, i have set the flexfield item as not updated. but the segments of the flexfield item still can be updated, could you please tell me how to make the segments cannot be updated?

for the post GL journal, the account cannot be updated, but for the unpost journal, the account updated, but i don't know how to do it

Thanks

Bruce