Security plugin: WordPress Simple Firewall

Advantages for you in free version:

  • brute force protection for login and user creation
  • blocks attempts to call WP procedures in URLs
  • blocks SQL injections in URLs
  • blocks attempts to check directory structure

You can set even harder rules for blocking but these options can be used only when you site has all plugins installed and you just add new posts and media files. Otherwise these harder rules can block installations.