ADD TO Game.js

-- buildSpackRocks
r.checkWorldBounds = true;
r.events.onOutOfBounds.add(this.resetRock, this);

-- ADD
resetRock: function(r) {
    if(r.y > this.world.height){
        this.respawnRock(r);
    }
},

respawnRock: function(r) {
    r.reset(this.rnd.integerInRange(0, this.world.width), this.rnd.realInRange(-1500, 0));
    r.body.velocity.y = this.rnd.integerInRange(200, 400);
},