Solaris manuals outline how to handle disabling a serial console. You can read up on them
here found at docs.sun.com. Earlier versions may use /etc/inittab for ttymon rather than the SMF.
Solaris 10: svcadm disable system/console-login
Solaris <10: comment ttymon in /etc/inittab
Note: I audit and secure Unix systems and I would not recommend disabling the system console. When it has to be secured it can be managed through a terminal server with 2-factor authentication.