Yes, you can always get a refund through your Advertiser Panel. If your campaign has already started the refunded amount will be for the remaining amount of traffic.
Please first cancel the campaign so that the funds are returned to your advertiser's Wallet, you can do this by clicking on the little configuration icon next to the campaign and then click on:
button. Now the transaction will be available in your Wallet either to create a new campaign or for a refund to PayPal by clicking the 'Refund' yellow button next to the Transaction Code of your campaign.
Or if your transaction was made before the 16th April 2013 - please contact us with the campaign reference