This is happening because anyone (or botnets) can access your WP Dashboard login page (1) …
… and you are using the default “admin” user in WP (2)…
… and there is currently a known bug in the iTSec plugin ban code which prevents the plugin from permanently banning ip’s.
In short your WP website is vulnerable to brute force attacks.
The many admin user lockout emails you are receiving is proof of such an attack. I hope you are using a strong password …
If not change your “admin” user password IMMEDIATELY to a strong password !
Then you need to create a new administrator user (with a user name that is difficult to guess), logout and login again with the newly created administrator user and then delete the “admin” user (2).
Deleting the existing “admin” user will allow you to transfer all existing content to the newly created administrator user.
Then enable the iTSec plugin Hide Backend feature (1). This way botnets won’t be able to find the WP Dashboard login page …
dwinden