The Greasemonkey script "No YouNoobs" claims to remove all YouTube comments -- unfortunately, the versions I've found were out of date, so I updated it: http://pastebin.com/eru2LVqp
Save it as anything.user.js (".user.js" is important), and open it in Firefox -- Gresemonkey will offer to install it.
You need the Greasemonkey extensions for Firefox installed (perhaps obviously). I haven't looked into it for Chrome.
