Assuming you're the buyer (sellers cannot cancel returns) try from My eBay > Messages. Click on the return message from ebay and then click on the blue "See return details" tab. Near the top right of the next page, below the blue "Reply to seller" tab you should see - "If you no longer want to return this item, you can close this return".
Don't close it if the seller has promised a refund without return. They can do that from the return. You won't be able to open another case.
If you still have problems you can ask ebay to do it using the live chat link on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
