E-mail your W...

W: Please use this message as a formal request for you to return the cell phone you have in your possession that is listed under my cell account. Please send the phone via the USPS by June 1, 2010.

If you can't cancel her phone due to the contract at least you will have it back and she can't use it further. Often times cell carriers have a clause about contract obligations if a "life change" takes place such as a divorce.