Why did the ghost decide to stop renewing the blood stains ? how had these blood stain become a joke for the otis family?

The Ghost had stopped renewing the stain, because Washington Otis would keep on removing the stain with "'Pinkerton's Champion Stain Remover and Paragon Detergent". Moreover, the Ghost had almost finished the paints in Miss Virginia's paint box. So he stopped replenishing the bloodstain and consoled himself, by saying that the family did not deserve it. The changing colors of the bloodstain was of interest to the Otis family. They even lay bets as to the color of the stain on the following day.