At the layout level, you can create script triggers that fire when such a field is entered and the script can check the lock field, moving the focus somewhere else if the field is locked. (This won't work well with pop-up/drop down formatted fields as you can enter and select a value before the trigger "fires".)
At the data level, you can move these fields into a table with a one to one relationship linking them to your original table. Then, you can use Record Level Access settings to control whether or not a user can access the fields. You can look up "Editing record access privileges" in FileMaker help, paying close attention to the section titled: "Entering a formula for limiting access on a record-by-record basis" to learn how to set this up.