The Terms and Conditions tell you that prepaid is not refundable.
Prepaid allows you to load a small amount to verify service before you put
a large amount of funds on your account.
Quoted from T-Mobile Terms and Conditions
Additional Terms for Prepaid Customers
Your T-Mobile prepaid Service account balance, if sufficient, or your active prepaid plan, gives you access to our prepaid Service for a limited amount of time; you must use your prepaid Service during the designated period of availability. To use our prepaid Service you must have a T-Mobile prepaid Service account balance for pay as you go service or be on an active prepaid plan. Service will be suspended when your account balance reaches zero and/or you are at the end of the time period associated with your prepaid plan. Monthly plan features are available for 30 days, however, depending on the time of day that you activate your Service or that your Service expires, your service cycle may not equal 30 full 24 hour days. Your monthly plan will automatically renew at the end of 30 days if you have a sufficient T-Mobile prepaid Service account balance to cover your prepaid Service plan before the first day after your service cycle. If you do not have a sufficient T-Mobile prepaid Service account balance, your prepaid Service will be suspended unless you move to a pay as you go plan. If you do not reinstate prepaid Service within the required period based upon your service plan, your phone number will be reallocated. The Charges for Service and the amount of time that Service is available following activation of your prepaid Service account balance may vary; see your Rate Plan for more information. Prepaid Service is non-refundable (even if returned during the Cancellation Period), and no refunds or other compensation will be given for unused airtime balances, lost or stolen prepaid cards, or coupons. You will not have access to detailed usage records or receive monthly bills. Coverage specific to our prepaid Service may be found at http://prepaid-phones.t-mobile.com/prepaid-coverage and differs from coverage related to our postpaid Service.
If I was you, I would keep using the service as a spare or give the sim to a family member or something since your stuck with it.