How to handle users that don't really read the answers they get?

You can not really help people who do not read.

I agree, I had the same problem.
Every reply I post is on public topics. I do not answer the PMs of those searching assistance for example.
It’s a waste of time, because I’ve learned that a public reply can be useful to a thousand users, a private reply no.

For those people that don’t read anything (answers already given, documentations, advices etc) I prepared a canned reply, gentle and complete with information and links that may be useful, with the invitation to re-read the discussion carefully.
Obviously if the user does not understand I do not continue the discussion.

Probably that user will ignore it, but the answer will be useful to most new users who will read that discussion in future.