So, just to further clarify – you’d like everyone to be able to use chat for direct messages, but prevent everyone from making channels?
And is it correct to assume that you don’t want “Channels” to show up in the sidebar, but you’d “Personal Chat” to continue showing up there?
I think right now this would take a combination of a CSS customization (to hide the Channels section) and a little trust with your moderators (tell them not to make channels). But hiding the channels section would also put one more hurdle in the mix for moderators who might forget any guidance you give them to not make channels, at which point, you’d only be concerned about moderators going out of their way to break policy, which seems like a problem in of itself.