exampleconf.yml

user:
  someone: NOPE
group:
  somegroup: YOU GUYS ALL SUCK
#default: uncomment this to set a default message

nope-login.rb

#!/usr/bin/env ruby
=begin
    Slightly advanced nologin shell
=end

require 'etc'
require 'yaml'

cfg = YAML.load_file('/etc/nope-login.yml')

$user = ENV['USER']

msg = cfg['user'][$user]

def die msg
    puts msg
    exit 1
end

die msg if msg

Etc.group do |gr|

    if gr.mem.include? $user
        msg = cfg['group'][gr.name]
        die msg if msg
    end
end

die cfg['default'] || 'No login for you'