I think this won't work properly, because STORY_CONSTRUCT checks only if an object/unit has been build, yet, but while you're building Boba your enemy could build him as well. Of course the chance that this happens becomes smaller if you set a lower build time. And then one last thing that I'm not sure about: You'd have to put a STORY_CONSTRUCT event and an event that disables the build option for the hero also in the AI's story files. I'm not sure if this works.