I too have a need for that. Once a record gets a certain status, I would like to be able to lock all changes to this record and it would become a history record. We used to have a way to "greyout" certain fields or all on a giving screen (record)
You can use Manage | Security to lock records on a record by record basis that includes the value of a field in the record. You can define a number field, named Locked in your table. A script can set the value to 1. Then an expression such as: Not Locked will control whether a user can edit the record or not. You can also set up expressions that compare a date field to Get ( CurrentDate ) to make the records lock on the basis of a date field. If you have a field called CreationDate, that auto enters the creation date, for example, you could use CreationDate = Get ( CurrentDate ) to limit access to the record to the day that it was created. (Playing games with your system clock, however, would allow a user to get around this lock.)
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 detailed description of how to set this up.