The only people who should be trying to log into your website are legitimate users. Anybody else is probably up to no good. This plugin attempts to make your life easier and bans anyone who tries to log in as the ‘admin’ user.
The idea here is that you’ve already followed instructions like these to change your default user name to something else.
Pop this guy into your plugin directory and enable it, and anyone trying to log in as ‘admin’ will be blacklisted forever, or at least until you go and let them back in.
Anyway, here’s the download link.
Oh, and if you ever screw up and ban yourself, I have instructions on how to fix it.