I guess that this journal does not even send out rejection letters. It if did, you could consider using this Rejection of a Job Rejection letter as a template for a reply:

Rejection of rejection letter

Dear Sir or Madame,

Thank you for your letter of February 9th. After careful consideration, I regret to inform you that I am unable to accept your refusal to offer me a job in your department.

This year I have been particularly fortunate in receiving an unusually large number of rejection letters. With such a varied and promising field of candidates, it is impossible for me to accept all refusals.

Despite your department's outstanding qualifications and previous experience in rejecting applicants, I find that your rejection does not meet my needs at this time. Therefore, I will assume the position in your department this March. I look forward to seeing you then.

Best of luck in rejecting future applicants.

Sincerely, <you>, Esq.

The contrarian in me would want to send him something so stupendously and awesomely good that he'd actually want to publish it but couldn't.

if you like academic jokes, check this other one: http://phdchallenge.org

So this is where I've been submitting my papers to...

If you like JofUR then you may also enjoy Rejecta Mathematica: http://math.rejecta.org/ (Note: it is in fact serious by the way.)

I can produce the exact same results from any Journal.