Donald Sterling is an all out racist.
The NBA Commissioner cannot punish Sterling because he is an owner, who is his boss, however the other owners can and should do something to get him out of the league.
This is not even Sterling at his worst, look at his treatment of Elgin Baylor, the history with him and Sterling were worse and then you have this
Largest Housing Discrimination In US History
What hurts Sterling is losing money, so the NBA could release all the staff and players from their NBA contracts and allow them to be free agents, this would hurt the value of the team, allow players to get away from an asshole, this could plummet the value of the team and he would sell. The owners could suspend him indefinitely and not allow him to oversee his team's day to day operations, which could cost the team.
If he remains an owner it is doubtful he could attract anyone with any kind of talent and plummet this team back to the worst franchise in all of pro sports.
It's the owners that have to punish, no boycotts, no complaints from any race, nothing but hitting him in the pocketbook will hurt him.
Whatever is done, Sterling will tie it up in the courts for years, he is extremely litigious.