Aperture doesn’t appear to have a true lock feature for preventing edits or metadata changes. The common workarounds are workflow-based:

• Use a keyword like "finished" and build Smart Albums around it. For example, exclude finished images from your working album and keep a separate Smart Album that only shows finished files. This won’t lock the image, but it helps keep completed photos out of your active editing view.
• Use ratings or another status system to indicate readiness. Some photographers treat a certain star rating as "ready for web" or otherwise complete.
• For output-specific finals, duplicate the finished version and make print- or export-specific tweaks on the duplicate.
• If you want something closer to a locked final, export the image at full resolution and re-import that exported file (often as a high-quality JPEG). That gives you a rendered "final" version that won’t carry all the original RAW adjustment flexibility.

So: no real lock, but keywords, Smart Albums, ratings, and exported finals are the practical Aperture workflows.