User Profile
Step One: Within Admin>Users, search for the user
Step Two: Select user profile
Step Three: Select 'No' within the 'User is a Collaborator'
Step Four: Click 'Save User' to save the changes
Remove Collaborator from Content
Step One: Search for the content that you would like to add a collaborator to
Step Two: Click on the content title
Step Three: Navigate to the content Settings
Step Four: Click the Trash icon to remove access
Step Five: Click the Update button to save the changes