private_setters.rb

class A
  attr_reader :fish
  attr_writer :fish
  private :fish=

  def set_fish
    fish = :carp
    nil
  end
end

class B
  attr_reader :fish
  attr_writer :fish
  private :fish=

  def some_public_method
    self.fish = :chub
    nil
  end
end

#> a = A.new
#=> #<A:0x00000103482d90>
#> a.fish
#=> nil
#> a.fish = :barry
#NoMethodError: private method `fish=' called for #<A:0x000001034d3f10>
#> a.set_fish
#=> nil
#> a.fish
#=> nil

#> b = B.new
#=> #<B:0x00000103482d90>
#> b.fish
#=> nil
#> b.fish = :barry
#NoMethodError: private method `fish=' called for #<B:0x000001034d3f10>
#> b.set_fish
#=> nil
#> b.fish
#=> :chub