Default settings are to not terminate, kill or remove any session that a user initiates. Please check what you have for the following server keys.
#
# Allow NX Server to terminate oldest disconnected sessions:
#
# 1: Enabled. Enable the automatic kill of the disconnected
# sessions.
#
# 0: Disabled. Disconnected sessions are never terminated.
#
# When this option is set and the maximum number of concurrent sessions
# has been reached, the server will kill the oldest disconnected sessions to
# make room for the new session.
#
#EnableAutokillSessions 0
and this:
#
# Set for how long the server has to keep alive virtual desktops in
# status disconnected. When the time is expired, the server will
# terminate virtual desktops if no user are connected there.
#
# 0: Virtual desktops in status disconnected are never terminated.
# This is the default.
#
# >0: Keep Disconnected session alive for this number
# of seconds.
#
#DisconnectedSessionExpiry 0
If they are both set to zero, tell us what NoMachine product and version you are using.