Blacklist user agents controller concern

user_agent_helper.rb

module UserAgentHelper
  extend ActiveSupport::Concern

  Browser = Struct.new(:browser, :version)

  SUPPORTED_BROWSER_BLACKLIST = [
    Browser.new(UserAgent::Browsers::InternetExplorer.new.browser, UserAgent::Version.new('7.0'))
  ].freeze

  included do
    helper_method :unsupported_browser?, :user_agent

    def user_agent
      @ua ||= UserAgent.parse(request.user_agent)
    end

    def supported_browser?
      SUPPORTED_BROWSER_BLACKLIST.none? do |black_listed_user_agent|
        user_agent <= black_listed_user_agent
      end
    end

    def unsupported_browser?
      !supported_browser?
    end
  end

end

To blacklist particular browser versions in rails. Place the above concern in app/controllers/concerns/user_agent_helper.rb and include UserAgentHelper in whichever controller you need it. You can then use if unsupported_browser? to conditionally display a warning, flash or whatever in the controller or a view.

To blacklist further browsers/user_agents just add a new element to the SUPPORTED_BROWSER_BLACKLIST constant.
e.g.

Browser.new(UserAgent::Browsers::InternetExplorer.new.browser, UserAgent::Version.new('7.0'))

see useragent gem for ::Browsers you can specify (you can just use a string instead). For the version comparison to function you must use UserAgent::Version.new('n[.n.[n]]'), rather than a string, for the version attribute.