Hi,
Had exactly your scenario a few days ago - a wordpress site got hacked and started to send out LOADS of junk...
don't think you can set it on a user/mailbox or domain level. As it's a script, mailbox limits don't apply. Would have thought domain limits should apply but they also didn't catch it, *but the subscription limit caught it* !
Running CentOS 6.6, Plesk12 latest update, and have the following defaults for outgoing mail control configured: 500 per mailbox, 1000 per domain, 1000 per subscription. Serverwide. Quite high, but I only want to catch scenarios as yours or mine...
Hope it helps
TomBob