World needs to set to creative(might happen in survival too), with the world type set to normal and cheats on.
First to /tp username 2999999 256 ~ (the y can be anything you like but I use 256 to be safe when in creative so I don't have to worry about spawning in blocks and talking my time to get back to the surface)
Now wait a few seconds.
You should get kick out of the world with the error of illegal position, this will also make the world unplayable without using 3rd party tools to move the players location.