Send them a letter rejecting their rejection letter and show up anyhow. From gettingintogradschool.com:

    Thank for your letter of April 9. After careful consideration, I regret to 
    inform you that I am unable to accept your refusal to offer me admission 
    to our 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 W.M.U.'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 join the ranks of graduate students in your 
    department this September. I look forward to seeing you then.

    Sincerely,