You can create a server task to automatically purge the Threat Event Log.
For details about product features, usage, and best practices, click ? or Help.
Select Menu → Automation → Server Tasks.
Click New Task, Server Task Builder appears.
Enter the Name and Notes(optional). Next to Schedule Status, select Enabled, then click Next.
Select Purge Threat Event Log from the drop-down list.
Select whether to purge by age or from a queries result. If you purge by query, pick a query that results in a table of events.
Click Next.
Schedule the task as needed, then click Next.
Review the task's details, then Click Save.