tmux kill-session -t sessionNum
pkill -f tmux
# from outside of tmux
tmux kill-session -t <session number>
# from inside of tmux
command: CTRl-B and after that press : (colon)
kill-session -t <session number>
tmux list-sessions | awk 'BEGIN{FS=":"}{print $1}' | xargs -n 1 tmux kill-session -t