lecture at (one)
To give one a stern talk meant to educate or enlighten. Every time my parents start lecturing at me about fiscal responsibility, I just try to tune them out. Instead of just lecturing at your staff, why don't you try leading by example?
lecture at someone (about something)
to talk to someone about something in the manner of a lecture. There is no need to lecture at me about the problem. I know how serious the matter is. Don't lecture at me all the time!