The strategy I've adopted is to first be polite by writing "hey" back [0] and then helping them with whatever they were asking. But THEN I end the conversation with a link to nohello.com to politely ask them to just directly ask next time. That's not being difficult but has the potential to improve things over time.

[0] If I'm in a bad mood I reflect the exact sentence back, meeting "hi" with "hi" and "Hey how are you?" with "Hey how are you?", but still, it's a reply..