Chances are, if you just wait a while the connection will be dropped.
What is the /whois information of yourself and the zombie? Are they the same? Maybe just wait a while and it will go off line. Also you could try closing your mirc and reopening it.
Another possibility is that maybe someone noticed you changed your nick and quickly moved in and registered the nick. You could try multiple ircopers and csops to explain that it was your nick first.
