PSA rebuttle: Doesn't matter if you're not a dick
Unfortunately, a large portion of the community are the kind of people who will not accept advice in any form. They'll report you for "negative attitude" or "harassment" because you told them that Thresh should be a tank, even if you tell them you love them along with the message.
So it doesn't matter whether you're a dick about it or not. Just don't help people. Chances are you'll get reported either way.
*** If you can, help them AFTER the game. As long as it wasn't ranked, the loss they cause won't mean much.