In the game there is alot of unused land. Maybe there should be a cheaper variant of settler (colonist?) to claim it which would allow to link up resources and deny passage but have no citizens to work on tiles (or capped at 2?). Resources, especially bonus resources, should have more use. Example is my Food Resources mod for Civ5: there are food buildings which consume bonus resources (like factory consumes coal) so the player is interested in getting more of them in his empire.
Some special unit like a Great Explorer may have an ability to occupy a large body of unowned land.