How could you disable the Failed Logon alert in EM as it is causing very heavy loads

Failed Logon is a delivered metric by EM 13.3c.  Because our aud$ table has a few million rows (less than 10 Million), this statement is taking several hours to run causing huge disk reads and buffer gets.  It is set to run every 30 minutes.  I have tried many things to disable this event and all of the attempts have failed:

1) Tried to disable this event altogether
2)  Set it to run ever 999999 minutes so it does not run very often

If the only solution is to trim the aud$ table, please let me know the safest way.  We still want to keep at least one year's worth of data in it.