I would just like to state, if it somehow has not been stated in the past, that this theory, if true, could be proven in an extremely short amount of time.
If the earth is indeed a flat disc, surrounded by walls, one could simply travel to those walls, and witness a vast nothingness on the other side could they not?
A plane could simply fly past these walls, and continue on past the end of the earth.
Any legitimate society should be able to gather the funds for several people air fare and equipment for what would be a very short expedition to do so.
I would love to hear any tangible evidence as to why this is not possible, but any person, being as open minded as humanly possible, cannot deny that this would prove the theory correct instantly.
If you are going to support such a theory, you should support such a quick method of actually proving it.