Good heavens, how do you send SIGQUIT to your programs then?
But if for some reason I wanted to do that, I would use C-\ : send-prefix. That sends the prefix character to the process running in tmux, in this case the shell. It's also super useful if you ever find yourself running a nested tmux (i.e. in an ssh session) with the same prefix character. You can just send-prefix d to disconnect the remote shell from the remote tmux session, then open a new local shell outside of your local tmux session, log in and reconnect to the remote tmux.
You can open a new shell and reconnect to the remote tmux first, then use Ctrl-b, Shift-D to bring up a list of connected sessions and kick off / detach the nested original one you don't want.