class Loofah::Scrubbers::NoFollow

scrub!(:nofollow)

:nofollow adds a rel=“nofollow” attribute to all links

link_farmers_markup = "ohai! <a href='http://www.myswarmysite.com/'>I like your blog post</a>"
Loofah.html5_fragment(link_farmers_markup).scrub!(:nofollow)
=> "ohai! <a href='http://www.myswarmysite.com/' rel="nofollow">I like your blog post</a>"