Currently there is no way for you to combat or revenge wagon theives if you dont have the time to escort your carts. You send them back to the capital they will just come back a few seconds later. Border camping is the only thing you can do and is not viable as a solo player as cities have multiple entrances.
I do understand that stealing carts i the only way for losing kingdoms to earn money after all the resource points near there capital have been taken but at the moment there is very little you can do to combat this.
Here are my soloutions.
1. Wagon theivies must use a hero stamina to rob a cart. If they have none avaliable they cannot rob a cart. This would make sending them back to the capital actually useful.
2. Sending people back to the capital could give them a 5 minute debuff on theiving wagons.
3. Players must be in a city to rob carts.
I would be happy if either one of these would be implemented.