There’s a site setting called
enable emoji, once that’s disabled they won’t be autoemoji’d. That’s a site-wide setting though, if you’re not an admin you’ll have to live with their decision to autoemoji!
Otherwise you can wrap your emoji in backticks like this: `:)` to get
It’s technically possible to spam ZWNJ (it also gets past the minimum length limit) but there doesn’t appear to be an IME that automatically adds the ZWNJ when inserting :( and friends.
Regardless, this is not something that should require an IME-level workaround.