How to block parent directory

We have FTP server. We have different clients. Clients uses FTP server to copy their files. Directory structure is as below
\
A (Sub directory under root for client A)
B (Sub directory under root for client B)
C (Sub directory under root for client C)
D (Sub directory under root for client D)
.
.

When client A login to FTP server he shouldn’t see any directory otherthan directory A. I denied client A access to other directory. But my question here is client A should not able to list other directory names (Ex B, C, D …). Is ther any logon writes we can deploy and how. Please send me the example script.
Thanks