You can go to the control panel and Domains==>Manage domains and click the [x] on the domain. That shouldn’t be necessary, however. Since it is just a mirror and not a fully hosted domain name, it’s just showing you what’s on the main site anyway. If there is some sort of problem, I doubt it would be resolved by you deleting the mirror.
The Insane Cabbit
My Blog http://www.sounanda.com
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Send me a pm if you want cms or forum software installed (for a fee)