Maybe one of the XML Spawner gurus can answer this better, but it seems to me that if you make it so that he always spawns with a spawner in his pack, and that on that spawner it has the static wall appearing at that location, that should do it. Because it would definitely go away when the Mobile did. As far as scripting it goes, despawning is the same as deleting, so if you create the static wall in the constructor of the Mobile, and delete the wall in the OnDelete() method of the Mobile, that should also do it.