Tim,
It would just be a matter of running a query on the Audit_Log Document Type, then deleting the records you need to purge. See below from our Help Files:
Purging the Audit Log
Depending upon how many database-related user activities you’re logging, and the size of your user base, your audit log can quickly become quite large. Fortis provides a tool to let you purge selected data in your audit log.
Do the following to purge entries from your audit log:
Create and run a Query, using the Audit Log Document Type, to find obsolete data in your audit log.
Select the audit log entries to delete.
Choose Audit Trail Purge from the Tools menu. Fortis asks if you to confirm that you want to delete the selected audit log entries.
Note: Purging records from the audit log has no effect on the original documents or data referred to in the log.