Registration is a year at a time, so once you've used that credit, you own that registration for a year - like it or not.
Hosting, however, is a different somewhat unrelated system. Only the WHOIS data ties registration to hosting. You can kill that domain, but that will only delete the hosting, and not the registration. With unlimited hosting here, you're not saving anything by killing a domain.
At $9.95/yr for domain registration, the free domain credit's nice, but it's certainly not why I host here. The $10 is a very small annual expense in the grand scheme of things.