You cant because the only things you can use to block are Trees, Bushes, and PWP's and all of those cannot be build close to a wall/cliff.
You can however create an illusion that its blocked off I suppose with enough trees,bushes and flowers.
Don't know. But I would bet money that the game would prevent you from doing this. I would be surprised if hedges would grow there. But who know's haha!
There is no way to do that. You would of had to of picked a map with a cut off beach with no ramp.
The most you can do is like I said, make it look like its blocked off.