At the "Select Player" screen, enter the "Eliminate Name" menu. Highlight "Exit", hold Down + A + B on controller 2 and press Start on controller 1. If done right, you'll have the option to delete all the save files at once. If you do so, press B at the "Select Player" screen to go back to the title screen and select "2 Players". While playing, all the blocks will be Line Blocks.