I have disabled emoji shortcuts on my site using the settings, and this works in posts, but they still get converted in chat. This seems to be a bug because there is no separate setting for chat.
I can confirm this too.
Is there any reason why you don’t want the autocomplete?
Many of our members prefer to show the ascii version of a smilie rather than the emoji, and if they want an emoji they can use the emoji picker or an emoji keyboard.