[Goad]

Instigate unrest (well, I've never been able to actually use that ability in any useful way, but I'm sure that's my fault and not the bards'.)

with ten or a dozen spies/bards in enemy capital, you can deprive him of his capitol-only unique units, and/or his biggest income province. The AI never seems to catch on to this trick. Put stone idols or that disease thing on them, too. Wipe out the population of his capitol with disease/unrest/patrolling & he gets no more or very limited special units for the rest of the game.