How do I force-close all active ssh sessions that were started by a specific sshd daemon?

if you have the pid of the sshd,

pkill --parent pid

should send SIGTERM to all children of the sshd

maybe add --signal kill if you want to force-kill them

Run this to find out which PIDs you're interested in.

netstat -n -eep | grep port_number

Run any of these commands to see the interesting processes children and terminate them:

ps fax
# or
pstree -p

If you know the ssh port, you can simply kill all current tcp sessions that were initiated through it:

ss -K dport 22 

This kills all active ssh sessions that were started through port 22 while leaving other ssh sessions intact. Don't forget to stop the sshd daemon before you kill off its sessions.