A better question would be why you want to do this?<p>If people use the Alexa toolbar it adds a header to their user agent you could use to block them[0], but Alexa is pure junk and not worth concerning yourself with[1].<p>I also don't think people quite realise the privacy implications of using that addon, it phones home on literally every page a user is on, that's how it works. Every single site, (porn) link, whatever is tracked.<p>[0] <a href="https://webmasters.stackexchange.com/a/72663" rel="nofollow">https://webmasters.stackexchange.com/a/72663</a><p>[1] <a href="http://www.closetnoc.org/?i=definitive-proof-that-alexa-sucks" rel="nofollow">http://www.closetnoc.org/?i=definitive-proof-that-alexa-suck...</a>