1. Navigate to your organisation (click on the building icon or organisation name in the top right of the menu, or hover over organisations > owned organisations and click on the name of your organisation).
2. On the left, click organisation settings > user settings > users.
3. Click on the user's name who you want to remove.
4. Add today's date under end date.

That user will no longer be able to access, or act on behalf of, your organisation. The end date starts at the beginning of the day.