Ok. You can add language that states that the fee is to cover a two year period, if the service is terminated by either party for any reason prior to the end of the 2 years period, that a pro rated amount of the fee will be refunded. You could also add a liquidated damages section, which would state that if they terminate the agreement, you will be entitled to liquidated damages of X dollars.
I can give some samples, but I am not allowed to draft your contract clause for you.
Termination and Refund. The term of this Service Agreement is 24 months. The agreed upon price for the 24 month period is X dollars. This agreement may be terminated at any time by either party.
A. Termination for Convenience: If A (you) terminates the Agreement for convenience, B shall be entitled to the entire fee of X dollars. If B terminates the agreement, the fee shall be refunded at a pro rated for the remainder of the term of the agreement.
B. Termination for Cause: If A terminates the Agreement because of B's failure to perform as agreed, B shall return the portion of the fee at a pro rated.
C. Calculation of Pro Rated Rate: The total fee of X dollars is for a 24 month period. Each month shall be 1/24 of the fee. The fee shall be earned on the 1st day of each month. In the event of a termination, the fee for that month shall be considered earned, and every month thereafter shall be refunded.
This is just a general sample... you will have to play with it to make it work for you.