deleting the default designs shouldn't be possible imo
Its not... well besides from tearing the game install library apart.
deleting your own custom designs should be much easier.
It really should!
i don't see much sense in them persisting across games, or the ai being able to use them.
I think its really fun that the AI uses my old unit designs against me, it broadens up the unit list for the AI the more I play the game, I would wish the AI would do on-the-run designs, or atleast copies of my unit designs I as a different faction, would make for a much more lively and changing AI.
Your customized sovereigns are also in the unit list... so take care if you don't want to delete those sovereigns.
Oh yes, sorry forgot to mention that