For Firefox, it's easy. Disable Flash in your default profile (the one you usually use), and then launch the browser as follows:
browser = Watir::Browser.new :firefox, :profile => "default"
Additional information: http://watirwebdriver.com/firefox/
For Chrome, I thought it would be done, but it looks like I'm doing something wrong, since the flash is on:
browser = Watir::Browser.new :chrome, :switches => ["--disable-internal-flash"]
Additional information: http://watirwebdriver.com/chrome/
, , Flash Chrome:
browser.goto "about:plugins"
browser.span(:text => "Flash").parent.parent.parent.a(:text => "Disable", :class => "disable-group-link").click