Open your keybindings on the slave accounts, and besure that it is set to save keybindings for specific characters. Unbind the Open Chat (Default: Enter), Slash Open Chat (Default: /), Reply (Default: r), and Re-Send (Default: Shift+R). This should make it so that you can type whatever you want on your main character, and your slaves will not say or do anything. You will still be able to switch to your slave and click a player's name or a channel number/name in your chat options to open up chat that way if you need to send a message from that slave.