(Note: If this is your own custom server without CPanel this explanation is not valid.)
The only solution I am aware of is provided by the CPanel ftp account setup used by the web hosting companies. All web hosting companies that I am aware of use Cpanel for the customer to set up their web sites.
While setting up the FTP account in Cpanel you are required to set the BASE (starting folder) for the user. The ftp user will only have access to the base directory and all its child folders (folders off of it).
For example if a directory structure is...
.-dir1--.
dir0--| |-dir2--dir3--dir4
| L-dir5
.-dir6--.
L-dir7
If userA has a base set to "Dir1" they will have access to dir1 thru dir5, but not dir0, dir6 or dir7. If userB has their base set to "dir2", they will only be able to access dir2, dir3, and dir4.
Also, I have never seen a way to allow access to just a parent directory but not the child. In my experience they get access to the entire branch of directories.
I am unaware of any other means of ftp directory control - and this is how I do it on all my web sites. I do not know if the .htaccess can in some way be used for ftp access. I also do not know if setting the group permission on a Unix file system could be used to control ftp access.
If you have Cpanel access and can make the one folder that needs access at the end of a branch your problem is solved :)