How do we report players for pre/post lobby chat?

Just use the normal report function in the post-game lobby. The lobby chat is recorded. The myth that it isn't came from the Tribunal because lobby chat wasn't presented to them, though Riot still had it.

If you can't report them for some reason or just forgot to, you can submit a Support Ticket and a Player Behavior Specialist will look into it. Providing screenshots and specifics doesn't hurt.