Crawl spam is a large volume of traffic from the search engine crawler. You can find out if your site is being visited by a crawler in your analytics. Spammers will dispatch illegal crawlers to scrape the content from different webistes. This type of traffic does not add value to the website. You can block these spam bots from the htaccess files of your website. This method is better than just blocking them in your analytics. You can download the Wp-ban plugin to blog these unwanted visitors by specifiying the criteria such as IP, host name, and referrer URL.