I've looked for this function too, but FMP does not have field level privileges (yet?). Anyone please correct me if this statement is incorrect.
What I've done when I was adamant about locking data field by field is to turn each field on the layout into a button which runs a script. Assuming the field is "Qty", the script would be:
If (Table1::Qty = "")
GoToField [ Table1::Qty]
Else this step probably not needed
Commit record this step probably not needed
In this way, the script checks to see if data has been entered, and if not puts the cursor in the field for data entry. If there is a value already there, you can't enter the field thus cannot change the data.
Note that, to swing the bat all the way 'round, you need to remove the fields in question from your Tab Order to prevent users from tabbing into the fields.
I've only done this with two or three fields and wrote a separate script for each. If you have many fields this approach may become prohibitive. Look into the SetVariable script step, Get ObjectName calculation function and GoToObject script step to see if you could make one generic script that you could put onto all of the fields.