A has many rails admin field with the remove association button gone.

not_removable_has_many_association.rb

require 'rails_admin/config/fields/association'

module RailsAdmin
  module Config
    module Fields
      module Types
        class NotRemovableHasManyAssociation < RailsAdmin::Config::
                                                           Fields::
                                                           Types::
                                                           HasManyAssociation
          # Register field type for the type loader
          RailsAdmin::Config::Fields::Types.register(self)

          register_instance_option :removable? do
            false
          end
        end
      end
    end
  end
end