I suspect that you need to use "TRUNCATE TABLE EventLog", be aware though, that this will remove all the records from the EventLog table, so you won't see any records under Admin|EventViewer.  You may wish to do a select count(*)  from EventLog before you do this, just to see how many records were there.  In future, you'll need to check Admin|EventViewer more frequently and clear down any log records you no longer need to look at.

In Host :: Scheduler you should enable the job to truncate the event log (PurgeEventLog) to make sure, it doesn't exceed size.