I don't know if it's time to throw up your hands yet. Have you tried the soft-sell? I've had this work before:

Start an alt of whatever class your problem player has. Get to level 15ish, then whisper them, "Hey, I was looking at your *spec/gear/rotation* for some tips, and then I saw this article on *pick your resource* and they suggested doing *something else.* I was wondering if you could tell me how you've found your way works? I'm all for trying new things, but I don't understand why they say to do things that way."

Or the old, I accidentally cut some of these gems, and it turns out they're not that great for me. I read where your class might use them better. Want a great deal?

Might turn them on to something they're doing/not doing that solves their issue. The disadvantage is you'll have to do a bit of homework and may be out some gold. The advantage is you're not calling them an idiot, you're calling someone else an idiot and making them feel like you think they have the superior knowledge.

Worked for me on a few occasions, but obviously YMMV. Good luck, and remember, holding back 9/24 people in order to placate one isn't fair to the 9/24. They commit their time and energy to progression, and everyone owes it to each other to perform as well as they can.