Looking at the site log, I found, that somebody with IP address: 22.214.171.124 is methodically sending requests to the page: /user?destination=node/add of my Drupal website. He does it once in every hour. It's definitely a bot. I think, he's trying to brute force the password. For now I banned him in .htaccess with
deny from 126.96.36.199
Can anybody give an advice, how to protect the site from brute force attacks on logins?