How do you restrict a user/collaborator from accessing a particular folder within Box?

Hi Beverly,

Welcome to the Box Community!

Was this user invited in the Parent folder of the folders you are pertaining to? If an individual is a collaborator on a parent folder, the access level is the same for all its subfolders as Box follows the "Waterfall Permission" model so it would be expected for him to see those folders if this is the case for this user.

You would have to remove the user from the Parent folder and then re-invite him only to the subfolders where he is necessary to be part of.

To learn more about Folder Permissions, please take a look at this article: https://support.box.com/hc/en-us/articles/360043697254-Understanding-Folder-Permissions 

Thanks for posting and stay safe!