You get powershell access to the exchange online pod but not sure if its possible.
@Zag - You can do it via Live@Edu once you have PS access, if the backend is Exch 2007 then use the same command, if it'x Exch 2010 then use the below
Search-Mailbox -Identity "Stident" -SearchQuery 'Subject:"Name of Subject"' -DeleteContent
@Chris/BandGeekMafia - Exmerge supports 2000/2003/2007, 2010 no longer supports Exmerge. If you run Exmerger against 2007 you have to do it on a management PC not from the Exch server, i.e which meets the sys requirements.