They could not clap the racist fans. They could publicly voice their disapproval of the fans’ behaviour. They could publicly request their fans to stop. None of these would affect their word cup chances. It may not make a difference, but if racists are supporting you then you need to do more than just ignore it.
I agree the banning should also happen, and for much longer. But unless the fans feel that disapproval from everyone, including the players they’re so strongly supporting, will it make any difference?