How do I unregister a subdomain?

I have a subdomain registered that for the life of me I cannot figure out the textual string for. I would like to unregister it so it disappears from my list of subdomains.

You can set its resolver and owner to 0.

I am unable to set the resolver to 0. When I try, the Save button remains greyed out:

I’ve filed a bug against our manager for this.