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.
Thanks for the colour coding hint Léonie and Kirby Krieger. This can probably alleviate the issue and I will try it out (and provide Apple feedback).
I certainly agree with the remarks on the relativity of the "final" attribute. In any case for me the locking feature would be of more value for intermediate versions in which certain adjustments have been finalised but more work is necessary. I have the situation that starting from an intermediate version I want to compare several alternatives of rather subtle further adjustments, each of which, however, may take (me) quite some time to perform. When looking at the different alternatives in the browser view, they are so similar that you hardly can distinguish them. Here it may easily happen that when one wants to continue to work, the wrong version is selected and you do not realise immediately. This may be particularly relevant when the work had to be interrupted for quite some time. Again I admit that systematic usage of color coding can help.
May be that my desire for a locking feature relates to my experience in the software field where you certainly want (and usually have the ability) to freeze your digital assets when you consider them to be in state that you want keep or return to, independent of whether you later derive further versions from those frozen assets. To me this appears to be a reasonable mechanism for any kind of digital asset library, even if it does not require too much time to recover from accidental changes.