I too just had the unfortunate experience of accidentally licensing and image that I had no intention of licensing, only to find out that this quick action can't be undone. I would very much appreciate a credit for this unwanted image especially since I am now paying for the service. The details of the unwanted licensing are as follows:
Sorry, yes, you can't revert a licensed item to unlicensed. You should contact Adobe customer care on this. Adobe customer care can be contacted here: https://helpx.adobe.com/contact.html - click the small callout at the lower right of the screen.