remove_method_in_class.rb

class A
  def f() 'A#f' end
end

module M
  def f() 'M#f' end
  def g() 'M#g' end
  def k() 'M#k' end
end

a = A.new
class << a
  include M
end

puts a.f # => A#f
puts a.g # => M#g
puts a.k # => M#k

class A
  remove_method :f
end

puts a.f # => M#f
puts a.g # => M#g
puts a.k # => M#k

undef_in_class.rb

class A
  def f() 'A#f' end
end

module M
  def f() 'M#f' end
  def g() 'M#g' end
  def k() 'M#k' end
end

a = A.new
class << a
  include M
end

puts a.f # => A#f
puts a.g # => M#g
puts a.k # => M#k

class << a
  undef :f
end

puts a.f # => undefined method `f'
puts a.g # => 'M#g'
puts a.k # => 'M#k'

undef_in_module.rb

class A
  def f() 'A#f' end
end

module M
  def f() 'M#f' end
  def g() 'M#g' end
  def k() 'M#k' end
end

a = A.new
class << a
  include M
end

puts a.f # => A#f
puts a.g # => M#g
puts a.k # => M#k

module M
  undef :f
end

puts a.f # => undefined method `f'
puts a.g # => 'M#g'
puts a.k # => 'M#k'