To make changes to a collaborator go to the Project Dashboard tab and click the edit symbol next to his or her name. If a collaborator has not entered any time to your project you can completely remove them from the Projects panel; otherwise, you may remove their active status:

Deactivating a team member prevents that person from logging any further time slips toward that project. You will still their past activity on the project, but they won’t be able to create new entries.