This has happened to us once and the fix was easy enough. If the users are getting this error when trying to connect to the Leopard FTP server:
FTP LOGIN REFUSED (access denied)
The solution is to remove the ftp settings and start over.
sudo serveradmin stop ftp
cd /Library/FTPServer/Configuration
sudo mv ftpaccess ~/Desktop
sudo cp ftpaccess.default ftpaccess
sudo serveradmin start ftp