how to mass delete after a hacked mail?

[voytek at sbt.net.au]

I had a hacked user account resulting in several thousand bounces back to
that email inbox, account now corrected, but, how can I mass delete
bounces with say subject like:

"Undelivered Mail Returned to Sender"
(by from ? ""Mail Delivery System" <MAILER-DAEMON at emu.sbt.net.au>")

or what other ways to clean the mess ?

thanks,

V