How to remove single package from cache/nix store

I prefer to use nix-serve to share packages between laptops

% nix-env -i nix-serve
% nix-serve -p 8080

more options how to share nix packages between machines described in nix manual

NixOS community has three manuals, always consult them first, if you're stuck:

• Nix manual, for the package manager
• NixOS manual, for the operating system
• Nixpkgs manual, for Nix package infrastructure

Every package on Nix is specified by a Nix expression. A Nix expression is some text, written in Nix language, typically residing in a file with extension .nix.

Every expression has the so-called “symbolic name”, a human-readable name that is printed, when you use nix-env. See sample Nix expression. Nix itself doesn't use this symbolic name anywhere internally, so it doesn't matter if your package is named aspell-dict-en, it's just for your, human's, convenience.

What actually matters is the so-called “attribute path”. So your confusion is between symbolic name and attribute path. Every package has an attribute path, which you can use in environment.systemPackages configuration option to install system-wide using declarative package management.

To find out your package's attribute path, add another flag -P to your query:

$ nix-env -qaP 'aspell.*en'
nixos.aspellDicts.en  aspell-dict-en-7.1-0

You should be comfortable using nix-env on a daily basis, so practice calling nix-env with --query and --install options. However you can also browse packages and find out their attribute paths online on Nix packages search. Type aspell, click on aspell-dict-en and you'll see various package's properties, including attribute path as part of the install command:

$ nix-env -iA nixos.pkgs.aspellDicts.en

Now you can put this attribute path into /etc/nixos/configuration.nix:

environment.systemPackages = with pkgs; [
  aspellDicts.en
];

Then update the system by running sudo nixos-rebuild switch.