When I pressed Ctrl+b w, I saw the windows I used before. How could I delete all of them?
From the tmux man page:
kill-window [-a] [-t target-window] (alias: killw) Kill the current window or the window at target-window, removing it from any sessions to which it is linked. The -a option kills all but the window given with -t.
So, at your command prompt try:
$ tmux kill-window -a
If you have one window that you want to preserve include it as well:
$ tmux kill-window -a -t:<window number>