After clean installation via NixOps, I logined to machine and executed
# nix-channel --update
This pulled ~130Mb of nixpkgs into my /nix/store. How can I undo this?
On a normal Nix or NixOS install, if you've updated your channels by accident with nix-channel --update, you can undo this by running
nix-channel --rollback
See the manual for more details.
On a NixOps installation, channels are configured but not actually used. See the other answer for that.