You will not receive any refunds for months already paid. You must return all videos by the end the membership period or they will become subject to additional rental fees.
Below are the official details for Movie Pass cancellation from Blockbuster.
Cancellation by You:
The initial monthly Pass Membership fee is valid for an initial Pass Membership period starting on the date of purchase and continuing through the same day of the following month. For example, if on June 18 you purchased your Pass Membership, your initial Pass Membership fee entitles you to Pass Membership benefits until July 18 and your Payment Card will be charged on July 18 for the period of July 19 through August 18. You can cancel your Pass Membership at any time at the participating store where you bought your Pass Membership and you will not be charged for any additional months. There are no refunds for any months (or portions thereof) already billed or paid for. For example, if your purchase anniversary date is the 5th of each month and you cancel your Pass Membership on Oct. 20, then you will not be charged on Nov. 5 (for the Nov. 6 – Dec. 5 period) and your Pass Membership Term will expire on Nov. 5. However, if you canceled on Oct. 2, then your Pass Membership Term would expire Oct. 5. Remember, you must return any Pass Rentals rented under your Pass Membership by the end of your Pass Membership Term (or they will be subject to the Rental Terms and applicable Fees as provided above). You agree that the Pass Membership does not involve an extension of credit or a retail installment sale since Blockbuster does not impose a finance charge and you may cancel your Pass Membership at any time.