how to mass delete after a hacked mail?

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@emu.sbt.net.au")

or what other ways to clean the mess ?

thanks,

V