In spite of the facetious replies you have had to this question it can be done. I have seen two photographs, one of a Kenyan and one of Indian sardu who had knotted their penises.
I do not know how the African had stretched his dick or why but i read in a book that came out some years ago, Modern Primitives, that some hindu sardus deliberatly stretch their penis to extremes as an act of sexual renounciation. A penis that is 18 " long when flaccid can never become erect. Once the man has extended his penis to such a dgree he may well tie a knot in it, firstly to add to the sexual mortification and secondly perhaps to keep it out of the way.
Others go to the other extreme and push their penis into the body cavity and seal it in with clay. They have a small hole to pee through.
I am intrigued by sexual size and extremes but for me this would definitely be going too far!