1) Open the GWAVA Management Console | Server/Scanner Management | <server name> | Manage Scanners | <scanner name> | Scanning Configuration | Text filtering | Subject filter/ Body filter/Subject+body filter (as appropriate)
2) Type the string you want to block in the add new filter box
3) Select what actions you want when the filter is triggered (check boxes on the right). Mousing over the icon will show you the action (e.g. Block, Quarantine, Notify, etc.)
4) Click the green plus to add the filter, then Save Changes at the top.