If all the domains you wish to delete are in their own realm, just delete the realm under Setup->Realms. Remember that you cannot delete the realm you're currently viewing. It's best to switch into the parent of the realm you wish to delete.
If the domains to be deleted are in a realm with other domains you want to keep, then you need to delete the domains' entries under:
Setup->Verification Servers Setup->Domain Mappings Setup->Authentication Mappings Setup->Domain Routing Setup->Realm Mappings
Be sure to delete the realm from Setup->Realm Mappings last, because that makes you relinquish control of the domain completely.