Now that you made look search for it, I find you are having the gardener, which is controlled by the player, add a snowman into the world. Remove what you added in your GreeneryWorld class as far as adding a snowman and change this line in the Gardener class:
to this:
if (Greenfoot.isKeyDown("f")){if (Greenfoot.isKeyDown("f") && getWorld().getObjects(Snowman.class).isEmpty()){