Secure file transfers

I am trying to setup an FTP server that will only accept file from one ip per customer. example customer A ftp's into customer's A account on the ftp server from ip xxx.xxx.xxx.12 customer B ftp's into customer's B account on the ftp server from ip xxx.xxx.xxx.10 I want to stop customer A ftp's into customer B's account on the ftp server from ip xxx.xxx.xxx.12 I do not want customer A to be able to get to customer B's account. I can deny access to all machines in the host.deny file and allow only xxx.xxx.xxx.10 and xxx.xxx.xxx.12 but customer A/B can still get to both accounts? It is possible to lock this down more? Thanks in Advance Dany Allard P.S. let me know if you need more info