Once you know Mag, you can traverse all water tiles, so it is not easy to blockade. I do not ever recall seeing an AI cut all the tiles for any town on any level, but I guess it could happen. That is something done by humans, not the AI. It will cut tiles on a town, if it is attacking and the town is too strong to be defeated in a direct attack. This is seldom going to last long enough to allow all tiles to be cut, but it could happen. I am not likely to be making any trades and certainly none for GPT in the modern age. It will be be a straight up tech for tech or tech for lux and if that is cut, I don't lose sleep. I am surely not going to be making GPT with a nation that is about to go under. I may gift them stuff. In short, if I am in the modern age I am only whacking them not trading. But then, I tend to just go to war with ever and stay at war, so rep does not come into play. My point was that you know what the game mechanics are for trade, so take care or take a rep hit.