byobu: How to force close a window?
Ctrl-a + k kills the currently focused window after a
y/n confirmation in screen/byobu.
You can use Byobu's hotkey, Ctrl-F6 to force-kill windows or panes.
Full disclosure: I am the author and maintainer of Byobu.
If you are using byobu in Emacs mode, meaning that Ctrl-a is mapped to "start of the line" you can use F12+k instead. F12 always works as the tmux/screen escape.
You will be prompted with a yes/no option from there.