// add field
private boolean spaceDown;

// for detection
if (getWorld().getObjects(Fireball.class).isEmpty()) // level not active
{
    if (spaceDown != Greenfoot.isKeyDown("space")) // change in state of "space" key
    {
        spaceDown  = ! spaceDown;
        if (spaceDown == false) // on new state  (change false to true for response on key press instead of release)
        {
            if (getWorld().getObjects(Scroll.class).isEmpty()) // scrolling done
            {
                // add fire ball
            }
            else // scrolling not done
            {
                // next scroll (or removal of final scroll)
            }
        }
    }
}