Head Strong Toddler

My daughter is 14 months and loves messing with plugs. We have plug covers over all empty sockets, however she still messes with the sockets that have things plugged into them (i.e. TV, cable box, etc). I sternly tell her "NO!" every time she goes toward the plug or touches it, but she just does it again and looks back at me as if she's testing me. I've spanked her hand (not super hard) and she still does it. I've tried to explain that it will hurt her...she could care less. I've redirected her to something else like a book or toy and she just goes right back. Idk what to do, I feel like a broken record. She clearly knows it's wrong, but idk how to get her to stop.