“Sorry, your belaying is innapropriate”. Whether the belayer has 1 day or 10+ years of climbing experience, just say it when you see it:
http://eveningsends.com/climbing/new-rules-belaying/
– Don’t take your brake hand off the rope
– The rope should never sag down below the height of your knee
– Don’t talk to the belayer
– Don’t talk while belaying
There are some people around who refuse to learn the appropriate use of the Grigri. Gently share this link with them if saying it at the crag/gym is too confronting. If you start receiving a few of those links, learn and belay properly!