How to remove/delete/destroy class variable in Ruby 1.9?

Ok, code below shows better what I am talking about, I understand that
@@var is acting like global variable.
But how to get rid of it when is not needed anymore?

class A
  @@var = 'var in A'
  def self.item
    @@var
  end

  def self.item=(var)
    @@var=var
  end

end

class B
  @@var = 'var in B'
  def self.item
    @@var
  end
  def self.item=(var)
    @@var=var
  end

end

A.item =>  "var in A"
B.item =>  "var in B"

A.item='test'

A.item =>  "test"
B.item =>  "var in B"

@@var ='ltl fkr'                #here is the funny part

A.item         => "ltl fkr"
B.item         => "ltl fkr"

A.item='test'                  #and here is getting even funnier

A.item         => "test"
B.item         => "test"    # i wish it to stay 'ltl fkr'