This kata's solutions have the answer to how craecke did it: http://www.codewars.com/kata/method-lockdown. Remember that ** is a method.

@Zedenem

mlabrum's solution works as far as I can tell, just add his code to the "Solution Setup" part and you should be good to go, unless I'm missing something...

I tested it and it successfully blocked it

I haven't done much with locking down methods or operators here, so what follows is just a shot in the dark:

undef **
class Bignum
  undef **
end
class << Bignum
  undef **
end
class Complex
  undef **
end
class << Complex
  undef **
end
class Fixnum
  undef **
end
class << Fixnum
  undef **
end
class Float
  undef **
end
class << Float
  undef **
end
class Rational
  undef **
end
class << Rational
  undef **
end

Does that work or help at all?