I clearly didn't explain this idea very well. OK, let's imagine we have a mains pipe of 3 inches in diameter sticking out of the ground on a building site gushing water which we can't shut off, but fortunately it's at least fixed in place. That's our high pressure source of unwanted liquid, analogous to the oil gusher.
As you say, sticking something into it or even over the end of it to create a seal is not very practical. But suppose we took an oil barrel, which is about 2 feet in diameter, and drilled a hole in the bottom big enough to fit over the pipe. Getting in place would be a bit of a hassle but once you got the hole lined up with the pipe it wouldn't be too bad, right? OK water is still gushing but now it appears to be gushing in a narrow jet from the center of the barrel resting on the ground. It's still coming out of the same 3 inch pipe, rather than the whole mouth of the barrel.
We anchor the barrel with steel cables or whatever we can. Maybe we seal around the bottom of the barrel with concrete, so it's well and truly fixed in place. Water is gushing out the whole time, but money's no object here. When we are sure the barrel is sturdily anchored, we start shoveling gravel or lead weights it as fast as we can. As it fill instead of gushing in a narrow jet, the water will start (quickly) filling up the barrel - it's the same amount of liquid at the same pressure, but in order to get through the much heavier lumps of rock it's now flowing through a much wider pipe, at a rate proportional to the circumference of the original pipe/the circumference of the barrel.
If we work fast, we can get some kind of plug - perhaps on with an open valve on it - into the end of the barrel before it fills. OK, so then the liquid comes gushing through the open valve. But that means there is not too much pressure around the edges of the barrel, so we seal them with more concrete (I am of course assuming that the walls of the barrel can handle whatever pressure of water is coming out of the original pipe).
Come to think of it, we might as well just drop the barrel with the open valve on top of the thing. The valve prevents pressure buildup inside the barrel while we work on making a seal between the bottom edge and the ground. It just seems a bit easier to me to use filler material to step down the flow from a narrow jet to a wider tube.
Now we still have a gusher, but instead of coming out of a small hole in the ground it's coming out of a big faucet which we have built around the flow without making too much effort to halt it. At which point we close the valve at the top and hope the seal between the ground and the barrel is stronger than the pressure of the oil.
