This module is enabled by default. To disable it use M-x customize-variable RET erc-modules.
/-commands which are not sent to the server are considered non-commands. Only the results of these commands are displayed in the buffer, not the command.
To add non-commands, add something like the following to your ~/.emacs:
(add-to-list 'erc-noncommands-list 'erc-cmd-FOO)