Achievements are turned off simply by toggling the "Creative" button in the main menu

-Create a survival world

-Save it (go in & out of world)

-Toggle Creative button, then toggle back to Survival (obviously don’t load into game in Creative!)

-Note: from this point, the “hasBeenLoadedInCreative” flag is now saved out to the level.dat file as “1“. This should only occur after you click “Play” and load into your world in Creative mode.

-Try to gain any achievements- it will no longer work