how I can give them some hints with soft words which can change their mind or at least give them some more space to think twice?
Some people are simply close minded, and there's nothing we can do with that. If I we're you, I'd just explain to them what a "ponzi" really means and how bitcoin isn't a ponzi, because it never promised any returns. Don't waste your time trying to convince people in to an industry that they probably couldn't understand.
Yea you are totally correct. I have some friends like this too, but some changed when they saw results, some other didn't. I think its best to leave the unbelievers alone else you will be blamed when they loose a penny. Let results speak