Block block = player.getLocation().getBlock().getRelative(DOWN);
MyTask myTask = new MyTask(block);
myTask.runTaskLater(plugin, 60L);

public class MyTask extends BukkitRunnable {
    private Block block

    public MyTask(Block block {
        this.block = block;
    }

    @Override
     public void run(){
        block.setType(Material.AIR);
     }
}