Try to train them...again, or simply shoot them?

Training won't do.  I say shoot them.

Try not to get overly worked up about it, whatjd. It's obvious that you know the risks and attempt to moderate them when loaning out your valuables. 

But here's the truth; soon it won't matter, literally. All the stuff you're working to protect - it will not matter. Someone else will dump it, sell it, use it and of course scratch, break it. So be it. You have to let go of the anger. 

There are two kinds of people; those who talk about helping others, and those who actually do. You, obviously, are a person who helps others grow in appreciation of love of music by doing, loaning. Others - they just talk about it. They will "give advice", but won't give equipment away (at least any equipment that's worth much), won't loan media. Big talkers, zero action. Let them cultivate their precious collection that in time - won't matter. 

Obviously, my priorities are a bit different than some. I have given gear away, loaned out discs and had them returned scratched, some go missing. My life has hardly been diminished. Frankly, I can't even remember the discs usually that went missing. Big deal. I am not living to die with the biggest, most perfect media collection, and I suggest you do not. Perhaps look into streaming audio; there's so much music coming out in so many genres you won't have time to cry about an old album or disc that was scratched. You can spend your time lamenting the attrition of the nostalgia pieces, or you can go get new experiences. 

Some won't like this advice; they will say it's "stupid", and the like. I just lost my father about three weeks ago. He loved rock, antique collecting, etc. Had to let it all go; the collectables, hobby and assorted paraphenalia doesn't matter in the end. I have one or two pieces that he gave me as keepsakes. There are potentially much different outcomes in things that do matter in the end when you cultivate your relationship with your family than keep your discs perfect. Whenever you offer to do for someone else, there is a risk of loss. You know that; it's obvious you're sharing frustration over the lack of concern that many have to return something in perfect condition. That is an unrealistic exception, given the variables involved. I think you know that, too. 

So, from one person who actually does loan, does give gear away, etc. I hear ya. But, remember (and this is a reminder for me as well, when damage happens), it won't matter long term. Seek out the new and ease the pain with fresh discovery and joy.   :)

I've learned the hard way not to loan out any type of personal property.

+1 douglas_schroeder

Stand a few steps back. It is just a CD and it is your son. You can buy a new CD, no matter how rare and precious it is. It is not worth being upset with your son over it. You have very limited time together.

Patient: "Doctor, it hurts when I do "this""...
Doctor: "Don't do dat"...
Problem solved.