Once you've deleted a channel, nobody, not even yourself, can ever create a channel with this name on this hub ever again.
Technically it's possible to do anything with mucking about in the database. That doesn't necessarily make it a good idea :)One problem I can think of is that other sites may not be willing to accept or send activities from/to a connection if it has changed it's public key. To fix this, you would (at least currently) also have to muck about in the databases of the remote sites as well.
This ability is a weakness of nomadic identity.