How to lock the x axis to make the player only be able to move down

so I’m having a bit of trouble locking the x Axis so that when you control the player you can only move it up and down.
here is my code so far that makes it move all over the scene:

    function setup()
-- Use Sprite Editor to import photos from your library (tap the highlighted text below)
    img = readImage("Documents:bg", WIDTH/2, HEIGHT/2, WIDTH, HEIGHT)
    player = readImage("SpaceCute:Rocketship")

function draw()
    -- spriteMode is CENTER by default
  sprite("Documents:bg", WIDTH/2, HEIGHT/2, WIDTH, HEIGHT)

-- you could use the image path directly, but loading it into a variable is better.
-- sprite("Documents:myImportedPhoto", WIDTH/2, HEIGHT/2, WIDTH, HEIGHT)

     -- Draw ground
    for i = 1,12 do
        sprite("Planet Cute:Ramp South", -WIDTH/18 -2 + 101*i, 6)
    sprite(player, CurrentTouch.x, CurrentTouch.y)

Just change CurrentTouch.x to some fixed value.

(Incidentally, take a look at the FAQ to see how to format code blocks. Or take a look at how I edited your post.)