I know pipes and doors need to be 2 way by default but if 2 invisible blocks were placed below a hanging pipe would that block the pipe? It is more of a question of weather the game will allow this.

anyone who can do this in gif/video form would be greatly appreciated.

The pipe is still 2 way, though in all likely circumstances the invisible blocks will permanently block it. The game has no way to stop you from doing this if you really want.

There might be some way to ascend Mario up to the pipe again without triggering the invisible blocks. If you're riding a Lakitu cloud or clown car, I forget if that triggers invisible blocks as you gain altitude, since you aren't technically "jumping".

It does trigger the blocks. But, you could fly in through the side, so I guess put blocks on the side too.

If you want to make it nearly impossible to go back, you could also angle the pipe horizontally and place it above the ground. Mario needs to be standing on something in order to activate a pipe, so a horizontal pipe with a drop like that should be one-way.

Or you can use the one way things to block the exit of the pipe, I have seen that and it works.

Another alternative is to have your pipe deposit Mario in front of a semi-solid platform, walled off (except top side) so player's only choice is to go back now or proceed.

You could also use that directional blocker thing.