[root@server ~] # cat /root/.plesk_banner
# This script is indirectly called by Bash on login shell startup.
# It outputs Plesk shell banner with login instructions.
# If you don't need it, you may comment out the following line.
/usr/lib64/plesk-9.0/ssh-banner || :
So, just comment out the line. Plesk won't restore the banner on updates.
Current Plesk is adding a line to "/root/.bash_profile" which is what includes the /root/.plesk_banner file, so you'll want to delete that include and then delete the .plesk_banner file as well. Disabling the SSH motd wouldn't make any sense (that just hides the problem) and the system /etc/motd file is empty.