After Janet and Michael accidentally revealed the nature of the Afterlife to Chidi, Elinor, Jason, and Tahani, Michael stated that their newfound knowledge meant that they could no longer earn any points towards entry to the Good Place because any positive action they took would be corrupted by the fact that they were doing it in an effort to get into the Good Place.
However, Michael later visits Doug Forcett, who discovered how the point system works in 1972 and has been trying to live an extremely good life in the interim. He's revealed to have 520,000 points and was briefly deemed by Michael to be a possible prototype for how to get people into the Afterlife, in spite of the fact that all of his actions are explicitly motivated by a desire to get into the Good Place (edit: or, at least, to avoid going to the Bad Place; whether that distinction is relevant is actually a fascinating question in and of itself).
Why was he able to continue to earn points after 1972?