How to Remove/Disable Sign Up From Devise

The easiest way is just removing :registerable devise module from the default list defined into your Model (the class name used for the application’s users, usually User).

class User < ActiveRecord::Base
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable

  ...
end

So you’ll have it like this:

class User < ActiveRecord::Base
  devise :database_authenticatable,
         :recoverable, :rememberable, :trackable, :validatable

  ...
end