Some useful stuff to wade through on this thread.
I never tire of the game, so any new theories are always well received.
I've got to admit to using the 'barricading wall of border control' to stop settlers getting in on 'my turf' - provided you can build the units quick enough. At the very least you need 5 units in the shape of a 'U' (3 in front, 1 to the left and 1 to the right of them) to prevent them advancing any further. But, yeah, finding a natural bottleneck helps.
I've noticed as well, if you have another CIV close to you at the start of the game, that you can keep them from building their second city for twenty or so turns if you can suss out where their settler unit (and spearman) are headed. Head for this square with one of your units and then 'dance' on and off this square each turn. Because the AI was programmed to build on this square if it is available, you can have them going somewhere else, then back again, then off somewhere else, then back again. Until eventually some other instruction kicks in that tells them to build elsewhere. By this time though, you've got several archers or warriors ready to attack their capital city (the only one they've built) - and as a result, force their 'new beginning' to crop up elsewhere on the map, well away from all the uncultivated land that you want to expand in to.
I also use the build cities 'round the coastline as well, whenever possible. But it's frustrating as hell when they sneak settler units in with caravels into the area where you've not quite finished marking as your territory.