Preventing bash from displaying “Done” when a background command finishes executing

My guess:

You have a password-protected SSH keypair, which is automatically loaded by GNOME Keyring when you login. However, cron does not have access to the keyring, and ssh cannot ask for a password either (due to lack of a tty).

To quote the ssh log you added:

debug1: Offering public key: /home/oompah/.ssh/id_rsa
debug1: Server accepts key: pkalg ssh-rsa blen 277
debug1: PEM_read_PrivateKey failed
debug1: read PEM private key done: type
debug1: read_passphrase: can't open /dev/tty: No such device or address

In future, try using && instead of ;
sleep X && echo bla pressing ctrl + C will stop sleep and will not echo bla

If it's feasible, you could rename the responsible binary . That way bash will not be able to execute nuke-russia since it will not exist.

Other solution would be to kill the responsible shell

EDIT:
How to find PID of responsible shell (which should its parent)
If pid of sleep 12h is 1234
find responsible shell with ps -o ppid= 1234
Then kill it with kill -9