You can use Manage | Security to set up a "lock expression" to limit the user's ability to edit records in a particular table on a record by record basis. Changing the value of your "lock" field can change how the lock expression evaluates for that record and users are now locked out of making changes.
See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a description of how to set this up.
Another approach is to set up a layout of global fields for creating a new record. A "Save" button runs a script that creates the new record and copies the data from the global fields to the new record. Users are allowed "edit" access to the layout with globals and not to the layouts/tables with the actual records created by the "save" script.