ruby 1.9.1p376 (2009-12-07 revision 26041) [i386-darwin10]
kronos:engine kronos$ irb
irb(main):001:0> c = Class.new { @@mvar = :mvar }
=> #<Class:0x0000010105d8c0>
irb(main):002:0> c.send(:remove_class_variable, :@@mvar)
=> :mvar
irb(main):003:0> exit
kronos:engine kronos$ rvm use 1.8.7

Now using ruby 1.8.7 p248

kronos:engine kronos$ irb
irb(main):001:0> c = Class.new { @@mvar = :mvar }
=> #<Class:0x100456a68>
irb(main):002:0> c.send(:remove_class_variable, :@@mvar)
NameError: cannot remove @@mvar for #<Class:0x100456a68>
	from (irb):2:in `remove_class_variable'
	from (irb):2:in `send'
	from (irb):2
irb(main):003:0>