MAC. How to prevent someone from remotely using say command.?

John D. gave you a good answer. Also make sure your Mac has a strong login name, you have the firewall turned on in the security section, and turn off all sharing except what you are required by your job to have on. The mute on is on the keyboard. You can also go into your sound preferences and mute your Mac there and require a pword to unmute it. also make sure you haven't accidentally turned on the voice section in the "universal access" control panel. It is to help the visually impaired.