app/models/user.rb

@@ -18,6 +18,8 @@ class User < ApplicationRecord
   validates_format_of :cn, without: /\A-/,
                            if: Proc.new{ |u| u.cn.present? },
                            message: "is invalid. Usernames need to start with a letter."
+  validates_format_of :cn, without: /\A(#{Setting.reserved_usernames.join('|')})\z/i,
+                           message: "has already been taken"
 
   validates_uniqueness_of :email
   validates :email, email: true

spec/features/signup_spec.rb

@@ -109,5 +109,11 @@ RSpec.describe "Signup", type: :feature do
         expect(page).to have_content("confirm your address")
       end
     end
+
+    scenario "Reserved usernames" do
+      fill_in "user_cn", with: "accounts"
+      click_button "Continue"
+      expect(page).to have_content("Username has already been taken")
+    end
   end
 end