Melkotian buoy

From Memory Alpha, the free Star Trek reference.

The Melkotian buoy was an orbital warning buoy placed by the Melkot, a telepathic and xenophobic race, to warn others not to encroach on their territory in space.

The Federation starship USS Enterprise made first contact with the Melkot through an encounter with a Melkotian buoy in 2268. However, Captain James T. Kirk ignored the warning and ordered the vessel onward. The Melkot then attempted to punish Kirk and members of his crew by trapping them in a bizarre recreation of the events leading to the gunfight at OK Corral, which were intended to lead to their deaths. (TOS: "Spectre of the Gun")