Slop and play in a mechanical system can never truly be eliminated. Here is where the law of diminishing returns starts to rear its ugly head. Pick all of the low hanging fruit, and hopefully the results will be good enough. From there it gets harder/more expensive to improve, and you will pay more for smaller and smaller increments of improvements. You may need to resort to scraping ways, improving ball screw mounts (the mounting bearings for screws are notorious sources of slop), higher precision ball screws (less worn).
(At some point you might be better off cutting your losses and getting a better condition machine, or just decide to live with what you've got.)
