So you ask them multiple times. You remind them via mail every n months. I repeat this is solvable. If people chose willingly ignore multiple warnings, then it's their fault.
Don't assume your users are immature just because they use a computer. This assumption is only with computers, I don't know why.
If the easy/simple first idea after giving it zero seconds of thought isn't good enough, the possibility actually exists to give it more than zero seconds of thought and try more than one thing.
It's also possible that the simple warning does work just fine, after it's ubiquitous for a while.
Everything about a computer was baffling originally, and now grandma scans documents to create pdfs and attaches them to emails. She's still grandma, she stil is baffled by many things, but there is a whole pretty big body of good understanding that she DOES have, just like she knows how to operate many other parts of her life.
The irreversable nature of protected data could perfectly well become one of these things that everyone knows.
All it probably requires is simply being a feature of ordinary life for some amount of time.
And maybe a little more standardization around terminology and ui so that people can tell when they are dealing with a secure thing vs an ordinary thing.
Don't assume your users are immature just because they use a computer. This assumption is only with computers, I don't know why.