There is a lot of really poor information and bad advice in this thread. By the terms of Spirit's own contract, in the event of a cancellation or a delay of 2+ hours, the passenger is entitled to a refund to his original form of payment. Period. Not sure where the others' information comes from, but this is directly from the contract.

You must, however, request that your ticket be cancelled and that a refund be issued.

While it is always possible that the carrier might not do this, it would be a very easy chargeback.