In a RAC Database :
SQL> ALTER SYSTEM SET audit_trail=NONE SCOPE=spfile sid='*'; System altered. SQL>
Note : Restart the database to have the change take affect, or do a rolling restart, one instance at a time.
In a Single Instance Database :
SQL> ALTER SYSTEM SET audit_trail=NONE SCOPE=spfile ; System altered. SQL>
Note : Restart the database to have the change take affect.