Unregistering a package

[Peter Simons]

I have an interesting problem. There are two versions of the
HsDNS package installed right now:

 $ ghc-pkg list
 | /usr/local/ghc-current/lib/ghc-6.5/package.conf:
 |     rts-1.0, [...]  (hsdns-2005-02-04),
 |     hsdns-2005-02-08

Now how can I unregister them? I have tried everything I
could think of, but no luck:

 $ ghc-pkg unregister hsdns
 | ghc-pkg: package hsdns matches multiple packages:
 |   hsdns-2005-02-04, hsdns-2005-02-08

 $ ghc-pkg unregister hsdns-2005-02-08
 | ghc-pkg: cannot parse 'hsdns-2005-02-08' as a package identifier

Can someone give me a pointer how to remedy this situation?

Peter