Comment on How to make sure your Linux system users won’t hide or delete their .bash_history / Securing .bash_history file – Protect Linux system users shell history by Milo.
To prevent saving session-history to .bash_history:
$ ps
PID TTY TIME CMD
13803 pts/4 00:00:00 bash
15368 pts/4 00:00:00 ps
$ kill -9 13803
Kill the login-shell process instead of logging out the normal way.