ftp user gets email too

I am probably doing something wrong, please help me out by showing my error.

I create an new FTP user using "Add a website FTP access user." and it works fine, his home directory is "/home/xxx/public_html" and all is good.

Then he checks email. Then he enters Usermin. Now i have "Maildir" and .usermin files and folders in the public_html folder. Convenient for scumbags, since the Maildir has the "here is your password" letter in it...

So, how do I keep "FTP" users from checking mail and using usermin?

Thanks!