I know that you can make an entire repository read-only for a project. In the Administrator Tool on the Lifecycles tab navigate to the project's baseline and this setting would be in the baseline configuration settings. Here is where you can read more about this: Administrate Projects and Lifecycles - CA Harvest SCM - 13.0 - CA Technologies Documentation
You can also control "view" access for any file or folder in the repository. In the Administrator Tool on the Repositories tab, find the repository containing the file, navigate to the file, right click and alter the "View" access list. Here is where you can read more about this: Item Path and Item Access - CA Harvest SCM - 13.0 - CA Technologies Documentation. The one problem with doing this is that it makes the item "invisible" for anyone who does not have permission to view it.
But I don't think there is a way in Harvest to make certain folders in a repository "Read-only" while still allowing "Read--Write" permissions for other folders in the same repository.
If there is a way to move these folders and/or files to a separate repository, that would be a way to make the files/folders read-only, since you could then make the entire repository read-only for the project.