After bitcoin is sent, the transaction is irreversible and cannot be cancelled. This is the way bitcoin was designed and applies not just to Airbitz, but to all other wallets and services as well.

You will need to contact the website, business, or person that you sent bitcoin to and provide them with your Transaction ID, which is like a receipt or proof of payment. Inform the other party know that you sent bitcoin to them, but would like to have it refunded. The Transaction ID can be found by tapping on the transaction from the transaction history screen, then tapping Advanced Details. At the Advanced Details screen tap on Transaction ID. This will open another window which is the relevant link to send, showing proof that the payment was made.