Just a quick pointer to a comment about deleting from aud$ that came up recently on the OTN Database Forum.
I know the answer’s in the manuals, but sometimes you just don’t spot little details, or remember where you think you read them, so it’s nice to capture the comment when you can. (And there’s a nice little follow-up from Mark Powell that extends the topic to fga_log$ – the “fine-grained audit” table)
Update Feb 2012
Here’s another OTN link to capture a reference on how to clean out the audit trail (aud$, fga_log$, or file system text of XML logs) “officially” and automatically in 11g using the dbms_audit_mgmt package. (The link currently points to Chapter 27 of the 11.2 PL/SQL Packages reference guide).