If you are at war, then putting a unit on a road will block the trade and cause devastation which apparently from what I have seen in the last year does damage the AI. Blockading ports with ships or by sitting on the road from the settlemant to the port works very well, as does destroying warehouses and markets, and then anything else available. Also eliminating the AI merchants will cripple them and eliminating their priests will help slightly. You will know when the tactic has worked because they will stop rebuilding because they are out of money and by seeing they are bankrupt on the Diplomacy screen. The AI is so unable to counter such tactics that this might be considered an exploit, but it sure does work well until the faction gets down to 3 provinces (from what I have heard) at which time it gets extra cash and seems to be able to still maintain existing large armies.