Like Kobe says, the ChangeTerrain trick is pretty versatile. It's been used to either cut down city production (i.e. eliminate mining) or eliminate cities (change city square).
Another related trick is to use Leo to 'upgrade' units into something completely different - Leo only requires the same role and mated obsolete/preq techs. For example, ground-slogging Frodo turns into a range 1 flying pickle (or a trireme far out on the ocean). Upgrade him when you know he's far from a city/base, and he'll crash. The crash text (@FUEL & @TRIREME in Game.txt) can then explain his disappearance, and Frodo II can appear elsewhere. If the setting's right, it's effective and uses very little event space, but ... it uses up unit slots, requires a special geography, and requires an exclusive use of air or trireme units.
Right now, I'm using Leo to 'upgrade' a hero to a nonhero unit (his leaderless followers). At the same time, a ReceiveTech trigger, linked to the same 'upgrading' tech, creates a new hero on the other side of an impassable barrier.
My explanation's kinda complicated (I'm tired

), but it's pretty easy to do.