I don't know of any lock mechanism in Aperture. I use color labels to alert me to the fact that a version must not be changed.
To finalize your edits you might consider to create a new master image capturing your version, either by editing the version in an external editor or by export-import., but that would break the connection to the original master.
This seems worthy of a feature request. "Aperture→Provide Aperture Feedback".
I use color labels as well. "Green" is fully developed. "Blue" is "Sold".
But I learned that it really is only the print that is "final". I regularly make small changes based on how an Image will be printed.
To me, a case can be made for an Image lock in Aperture to prevent accidental alteration, but it remains important to understand that digitization has in a profound way de-stabilized our notion of final.
I think, also, that the desire for a lock is more psychological than practical. In daily use of Aperture for two years, and the development and processing of tens of thousands of Images, I have only once had to spend more than two minutes trying to recover from an accidental change of adjustment or metadata.