Sorry for late update.
Set Delete Items Retention is a helpful way to disable "Recover Deleted Items" ,Thanks Adam.
But because I use powetshell to import PST file to Exchange , so set Delete Items Retention is not work for
this kind of message.
I found a way to clean up "recoverable items folder" :
Search-Mailbox -Identity "Administrator" -SearchDumpsterOnly -TargetMailbox
"Discovery Search Mailbox" -TargetFolder "Administrator-RecoverableItems" -DeleteContent
But it just for Exchange 2010.
So for Exchange 2007, how to clean uo "recoverable items folder" ?