A family of Microsoft word processing software products for creating web, email, and print documents.
John's suggestion is the best if you have a MS 365 Subscription & never intend to use Copilot.
If you simply want to disable it the instructions differ a bit from what Kudos-Ng proposed - The Ribbon cannot be modified in that way.
Go to Word> Preferences but in the Authoring & Proofing Tools group there is an explicit Copilot icon. Click that then clear the check on Enable Copilot.
The Copilot tool will continue to display on the Home tab but will be dimmed & inactive.