Daniel, there is an IDS rule that you can create on your system called "Internal Spammer". It will allow you to Notify an administrator, block the sender, or quarantine the messages. You can specify how many messages within a certain time frame will trigger the block and how long you want the block / quarantine to be in affect. It is located in System Admin > Settings > Security > IDS Rules. Below is an example of a rule you could set. Tailor it to your system and mail usage.