Can I just delete those events from the database manually?
Yes you can delete the entries from the "aiowps_events" table if you wish.
In the last update of the plugin we inserted some code which ensures that the "aiowps_events" table which logs 404 events will never exceed 10,000 entries.
This code runs once per day using the native wordpress cron functionality.
I am interested to know why your table was not automatically cleaned.
Can you please check the "wp-security-log-cron-job.txt" file and tell me if the following log is being produced:
DB Cleanup - checking if a cleanup needs to be done now
How can I keep the plugin from logging those errors?
You can turn off the "404 Detection" feature via the following checkbox:
Enable IP Lockout For 404 Events