Avoiding discussing a recommendation
A simple declination to discuss should suffice.


  Hi (Person),
  
  I will not be able to discuss (Mr/Mrs student). Sorry for the
  inconvenience.
  
  Sincerely,
  (Professor)


Don't make up a reason. It only gives someone an opportunity to continue asking if they have a way to mitigate said reason. Also, if it is indeed a nefarious as others have suggested, there is nothing for them to take and run with.