Not sure exactly what the intent is.
If you want this to impact deleted items only, you can turn on Page Zeroing on the Databases. This overwrites deleted database pages with a character, instead of the normal behavior of simply marking the page as deleted.
Online Maintenance Database Scanning in Exchange 2007 SP1
To purge items with particular keywords, you can use tools like ExMerge (pre-Exchange Server 2007, does work with Exchange Server 2007). For Exchange Server 2007, use the Export-Mailbox command. To ensure these items are not recovered from the dumpster, you can turn off Deleted Item Retention on the database(s).