Massive number of ‘Site Lockout’ notifications

A host was locked out for ‘Too many attempts to access a file that does not exist’. This is fine but the plugin started sending continuous notification emails – literally every few seconds – for the next hour until the Host mail server blocked it after sending 500 emails. All mails referred to the same host so it was not due to a DoS attack.

Clearly this is wrong – please advise how this can be blocked or fixed. I have not seen this behaviour before when hosts have been blocked for this reason.

The page I need help with: [log in to see the link]