Navigate to the folder or subfolder you want to remove a collaborator from within Box.
Click on the button to Change collaborator permission and choose Remove from the menu.
Remove a Collaborator from all Folders that you own
On the sidebar click on the menu item that says Collaborators.
To remove a collaborator from all folders that you own click on the Remove button to the right of the desired person in the list.
Note: for collaborators that are in folders you are not owner you will get a message when trying to remove that "You can only remove collaborators from folders that you own" and you will not be able to remove them.