Not sure exactly what the intent is.
If you want this to <b>impact deleted items only</b>, you can turn on Page Zeroing on the Databases. This overwrites deleted database pages with a character, instead of the normal behaviour of simply marking the page as deleted.
Online Maintenance Database Scanning in Exchange 2007 SP1
To <b>purge items with particular keywords</b>, 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).