From man tmux
:
unlink-window [-k] [-t target-window]
(alias: unlinkw)
Unlink target-window. Unless -k is given, a window may be unlinked only if it is linked to multiple ses-
sions - windows may not be linked to no sessions; if -k is specified and the window is linked to only one
session, it is unlinked and destroyed.
I think unlink-window -k
is what you're after.