You stopped your own processes without trouble. A process of another account is a different story.
A process takes signals from its owner or from root. For someone else's process, kill prints Operation not permitted, whatever the signal, KILL included. Such a process is stopped with sudo kill.
Take the process number from pgrep -a or ps right before the signal and check the command line: PIDs are freed and handed to new processes, and a mistaken signal reaches the wrong one.
After the signal, check the result. If the process is gone, ps -p PID prints a single header line. If it is still alive, it is either finishing its work or not answering TERM.
An export running as www-data has to be stopped, but kill 2345 printed Operation not permitted. What do you do?
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The running process does not stop because of that: the program is already in memory. To stop it, run
sudo kill 2345. -
In a new terminal you are still student, and the process still belongs to someone else. You need root:
sudo kill 2345. -
The refusal is about permissions: an ordinary user can send someone else's process no signal at all, and
kill -9prints the same.
A process takes signals from its owner or root, so someone else's process is stopped with sudo kill. You check the result with ps -p and the same PID.
Key points
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Signals to someone else's process go through
sudo kill; without sudo you getOperation not permitted. -
Take the PID right before the signal and check the command line, then check the result in
ps.