I had an AI novalith my moon with 2500 hp. After impact, it had 0 hp but was still mine because i had a starbase (well, 2 actually...) with enforced loyalty thingy. So, I'm pretty sure just having one at your homeworld will stop this, which imo, if you are playing with capital world victory on, you should probably have a starbase there anyway....seige ship rush by vasari superweapon would pwn too heh. And, although it isnt playable yet, that could be game breaking because you and your allies if you made phase pact could warp in entire fleets to their homeworld.....your allies being able to breing anti starbase whips and you could all be constructing starbases there/bring in titans.
Yeah, maybe outlawing superweapons would be best for capital world victory condition games.