well this is funny to read that some people can be lag free with a ping of 900 ms , it is strange cause i never notice this kind of fact in most game
like counter strike, or even warcraft 3 (although war3 is fairly good for 56k user but they lag at the beginning quite often) no player can play with a ping that high there will be obviously lag with such a high ping. And ping and lag are quite linked , i dont think u can play smooth if u need 200 ms to send a ping request ,so in a game it will be even worse.
Anyway i dont think we should ban the 56 k user that is for sure, the best thing would be to have low bandwidth netcode which is damn hard to make And also play on server located in or near ur country will help greatly to have a better connection