I can't seem to find any examples on the internet for this particular task.
Here is a simple solution, that will work if you are root.
su $uid -c kill SIGSTOP -1
Explanation: become that user, and kill everything that you can.
All solutions will require CAP_KILL (permission to kill any process), or to be traditional root (root has this permission), and permission (capability to change its own uid), as used by this solution.
Note I sent sigstop, this will pause the process (as asked for ☺). Chose the signal that you want. Use sigkill as last resort.
killall -u $user -STOP
This is safe if run as another user and (for other signals) if no parent process belongs to the affected user.
ctrl-alt-delor's solution is probably better (safer). This one has the advantage of giving you a real exit code, though. The other one probably does not as both
kill get killed themselves.