P2X-374 was a primitive planet in the Milky Way galaxy which held a Stargate and a group of primitive people who for some unknown reason displayed hostile intent toward SG-1.
As such, this ultimately forced SG-1 to return to Earth where Major Samantha Carter suggested to Major General George S. Hammond that the planet be classified as "unfriendly" which suggests that the planet's address was presumably locked out and permanently removed from the Dialing computer after SG-1 arrived back on Earth. (SG1: "Redemption, Part 1")