By killing, say, a few hundred of them using the twins like they did with the Empire? Pretty sure the emperor realized Ainz's power with that stunt. In fact, why didn't they just do that with the kingdom too? Actually, what's the point of spreading his name if he can just fly to the next country again and drop a dragon on them too?
Suddenly declaring independence which results in a war is not a passive or defensive action. They were the ones demanding stuff from the Kingdom, not the other way around.
Massacring hundreds of thousands of people is not just in their world either. That said, it doesn't matter if it's just or not, because Ainz has ABSOLUTE POWER.
Whatever, I quit.