I'm not convinced this is a problem.
I'm NOT sure how i feel about it.. just a suggest... cause now i realize you can do a lot of damage if someone doesnt have scum.. and then hit them after. Great strategy! but there is not preventing it (other than say keeping lots of scum)
The thing is, if someone is killing your troops like that, what's the stop them from just scumkilling you? You're correct, there's no preventing it save for more scum, and honestly I think that's how it should be.
They may want to scum your troops down then war you for the land, but there's a balance there. In order to take a kingdom that far down, how many turns do you have to burn scumming them, and then turn around and war them down, using even more turns? It's a whole lot more then just scumkilling them, or even just warring through their defenses.
There's always a tradeoff. A kingdom that is stacked that amazingly high with defense is open to scum. A person that kills off the troops could have just scum killed the kingdom. Or burnt the temples and sorced. Conversely had they carried enough scum to stop the scummers, they may have been too low on troops to stop a straightout war or suic-sorc kingdom.
This is really only coming up because of the massive size of some kingdoms, anyways. I'm not sure the numbers were ever meant to scale to such huge kingdoms, with so many thousands of turns to use. I'm keeping an eye on it though, to see how it goes.