I have two suggestions in this matter:
Firstly, a new diplomatic agreement which gives one civ's cultural boundaries precedence over another's, much like when you vassalize your neighbour. This would provide a step between a peace treaty and capitulation, but still be a prize worth waging a war for.
Secondly, and I think I've seen this suggested somewhere before, garrisoned forts have a cultural border. The extent of the border could depend either on the number of units in the fort, or perhaps upgrades to the fort performed by workers. Perhaps to balance this function the worker would have to be consumed?
Firstly, a new diplomatic agreement which gives one civ's cultural boundaries precedence over another's, much like when you vassalize your neighbour. This would provide a step between a peace treaty and capitulation, but still be a prize worth waging a war for.
Secondly, and I think I've seen this suggested somewhere before, garrisoned forts have a cultural border. The extent of the border could depend either on the number of units in the fort, or perhaps upgrades to the fort performed by workers. Perhaps to balance this function the worker would have to be consumed?