In my Iberian campain, I faced the spawning Celti Iberians (forgot the name). To deal with them I killed the army and then besieged the town in the same turn.

I waited out 8 turns for the town's army to become insignificant before I assulted.

So yeah, kill him and siege the town. He won't respawn.

But kill him, and don't reach the town and he will respawn.