Ok, to remove certain users from ssh login, do the following: You will need to be logged into the ssh accessed machine.
"sudo nano /etc/ssh/sshd_config"
Add the following line to the end of the document. Add or remove usernames as necessary. Do NOT include the actual "[username2], just put whatever users you want to not be able to login using ssh.
"DenyUsers Oracle [username2] [username3]..."
Save the file and then do:
"sudo service sshd restart"
That will disable Oracle, [username2], and [username3] from logging in using ssh. You can still, however, use "su Oracle" to change to Oracle after logging in with another user.