There is no uninstall command, there was at one point but it didn't do anything ;-)
Unregistering a package does not delete any files, but it will make cabal forget about its existence.
If you are working in a sandbox you can simply `cabal sandbox delete; cabal sandbox init` to start from scratch, you can also deregister packages there with `cabal sandbox hc-pkg unregister`, outside a sandbox it's `ghc-pkg unregister`.
There are also tools on hackage that can clean up files when unregistering, iirc cab[1] can.
HTH,
Adam