Hi,
Welcome to Microsoft Community.
I apologize for the inconvenience.
Unfortunately, in New Outlook and Outlook Web App, the emoji panel triggered by typing : is a built-in feature and cannot be disabled through settings at this time
You can try this workaround:
When creating a list or explaining something in New Outlook, avoid adding a space before the colon (:) character.
Instead of typing something like " Item : ", write it as "Item: "directly. This helps prevent the emoji panel from being triggered and replacing the colon with a smiley face.
However, I encourage you to provide your feedback requesting Microsoft developers to work on implementing this feature. You can access the feedback form through the link below:
https://feedbackportal.microsoft.com/feedback
Thank you for your understanding. Please let me know if you have any further questions or concerns
If my answer helps, please mark it as an answer, it will definitely help others in the community with similar queries to find a solution to their problem faster. On top of that, it helps me have a good survey and makes me do my best.

Looking forward to hearing from you.
Sincerely,
Kai Vo – MSFT | Microsoft Community Support Specialist