Hello! On my Website, I have "like" buttons (like Facebook) in CSS . When I look in Google's Webmasters tools, the 2nd keyword is "Like" and this isn't relevant at all to my Website. How can I prevent this keyword from being indexed? Or maybe should I leave this like it is right now?
Just leave it. It doesn't going to give your site anything harm. GA only shows you that from which sources you are getting the visitors and Like is one of them. So leave it.
The keywords display is the density of "each" words shown in Google's webmasters tools, not Google Analytic.