As you give us no information about what the hack does, what version you are running, nor a link to the forum, it is quite difficult to help you.
EDIT - Is this the forum in your signature? Where does the hack appear?
2d EDIT - I see you have restored a pre-hack backup so no one can now see the code or effects to do much to help. You may want to file a report with http://www.phpbb.com/incidents