The best start is to respectfully tell them that they need to improve, and then direct them to some good tutorials. If they get pissed at you, then...well, I dunno what to do after that, but...eh, it's just a suggestion :U
Unfortunately, that rule won't be followed, because most base users nowadays just don't bother to follow the rules :I It's sad, but you and I are part of a VERY small number of people who can be called mature base users.
Dude, these noobs doesn't know how to use the eraser tool. They just slap on colours using the round brush tool in Vista/XP version of Paint. They don't know how to handle the curve tool and doesn't try to learn how to use it. Some people even tries to do lines with the pencil tool, but they won't even clean their lines. And these people doesn't understand that you need to work hard whith pixelating.