I first read about this in George Takei’s Twitter feed. Then I checked it on Snopes, and it is true.
There really was a 1958 episode of a television Western (an episode titled The End of the World, of the series Trackdown), in which a character named Trump, a con artist, proposes to build an impenetrable wall to protect a town’s inhabitants.
Snopes confirms: the episode is real. In fact, the full episode is available on YouTube.