I had hundreds of spam daily on one of my blogs. Installed a simple free plugin that added a math captcha when posting comments. It was very basic, like "5 + ? = 10" and the spam was gone. Can't remember it's name but search for simple captcha or something, you won't need any hightech stuff to stop spammers from posting.
There are many WordPress plugins that blocks automated spam. Growmap Anti Spambot and Antispam Bee are one of the most popular WordPress plugins to prevent, detect and block spam in comments automatically. In order to stop spam attack logins and comments, you could also edit the .htaccess file in your WordPress directory or make some changes to your web server configuration file as described here.