With the new "unified" RC client app, the ability to post a code "snippet" is gone. (I'm not talking about the [code] ... [/code] option - that is different.)
With the old client, you could press Cmd-Shift-S (I think) to bring up a window where you could paste text into, and it would then be posted as a "snippet" without formatting.
This option seems to be gone now. We need a way to post text in RC without it trying to "interpret" the text and formatting it. [code][/code] works for some things, but Snippet was better in many cases.