If you need to remove an event-level collaborator:
1. Hop into the Settings tab of your event page.
2. Head to the Collaborators section.
3. Hover over the top right hand corner of the collaborator card and click on the button with the three dots that appears.
4. In the dropdown menu, click Remove Collaborator
5. In the window that appears, click on the red Remove Collaborator button.
6. Refresh the page, and this account will no longer have editing access.