Tmux: Map one default mapping to do nothing?
It depends on your tmux version, but tmux list-keys should show you something like
bind-key -T prefix C-n next-window
bind-key -T prefix n next-window
where -T prefix shows it is using the "prefix" table (the one got to by C-b). So you can try adding to your ~/.tmux.conf:
unbind-key -T prefix C-n
unbind-key -T prefix n