Install and use Akismet and Cookies For Comments plugins.
Try installing Akismet and do some moderation settings from Settings>Discussion.
Also try to use 3rd party commenting system like disqus https://ww.wp.xz.cn/plugins/disqus-comment-system/ It will prevent robot spam. For more details about how to fix this issue, I’ve wrote a tutorial here http://www.bloggingguts.com/deal-spam-comments-wordpress/
I´m using Antispam Bee. It´s one of the best Plugins to stop Spam. Very easy to configure. Take a look.
