One of the best features of tmux is the ability for multiple people to connect to the same session in order to pair. This can, however, sometimes result in a extra session hanging around if someone forgets to detach. This is no problem though because you can view and kill other connections.

Hit

<prefix>D

to open up an interactive list of all connections to the current session. Then navigate over the one you want to kill and hit enter. If you are viewing the connections but don't want to kill one, you can hit q to back out.

h/t Josh Davey