If you think you can help someone, try. If they ignore reasonable, grounded advice, just move on. In life or online, it's similar. There are threads I just ignore now, because they went off on the "I waited til I heard what I wanted to hear" or they took the "it works for 24 hours so it's great" approach. Why waste time fighting?
Discuss with people who discuss back. There's an enormous difference between a person who disagrees with you and is happy to discuss and maybe teach you something and a person who can't be bothered to question anything.
There are no obligations, except to be honest according to your own lights.
Yesterday, I was talking to man I had helped a lot when he was a teenager. He said things I thought were brutally selfish and predatory. Oh well. You offer what you can to people, and what they do with it is far beyond your control. You can't change people, just offer them the tools to change with if they care to. If they don't, it has as much to do with you as if they did. Sometimes, no matter how much you care, you just have to accept that.