In "Domains -> DOMAIN_NAME -> Setup -> Hard disk quota" the Hard disk quota is set to Unlimited. However, in "Domains -> DOMAIN_NAME -> Limits" a limit is setup, as well as in "Clients -> DOMAIN_NAME -> Limits". I am begining to think Plesk has too many different ways to set the disk space limit. It appears there is no limit unless it is set in the "Setup" icon. The other types of limits just change the control panel icon and send an email out, which we don't have time to deal with and the customer just ignores as they happily use up more disk space than they are paying for.
Plesk should fix this *bug* so that a hard quota is setup for every potential disk space user, including FTP uploaders, MySQL users, and, in most importantly, mail box users. The bigest problem is that there are tonnes of email accounts that are not being monitored, but are filling up with SPAM.
Thanks for your reply, Satoriya.