I had a similar problem whereby i needed to create an extra FTP for a client but lock him out of certain folders where an ASP application was installed.
1. In Plesk create a subdomain eg. "sub2.mydomain.com" making sure to select the option to give a separate user access to this domain eg: username: Username2
2. On the server navigate to the subdomain's folder eg.
C:\Inetpub\vhosts\mydomain.com\subdomains\sub2\
and remove all the folders except httpdocs
3. On the server using "junction" (from
www.sysinternals.com) map the httpdocs of the subdomain to the httpdocs of the main domain ie.
junction C:\Inetpub\vhosts\mydomain.com\subdomains\sub2\httpdocs C:\Inetpub\vhosts\mydomain.com\httpdocs
4. Go to the httpdocs folder (either one will do as they are effectively the same now) and add user "Username2" to the NTFS permissions using the original user for the domain as a guide.
5. To lock the client out of eventual subfolders remove user "Username2" from that subfolders NTFS permissions.
PLEASE NOTE: The reason I create a subdomain is because using another method and creating a dummy folder in the FTP root and junctioning that to the httpdocs folder I found that Plesk deletes folders (and the foldes they junction to) that it is not aware of. This is probably to make it difficult to manually add new hosting accounts without going through Plesk and possibly exceed liscence restrictions. In this way most of the setup is included in Plesk backups. Anyway the solution outlined above works a treat.