You can always `nix-store --delete /nix/store/path` if you don't keep the path alive by some references, but you should note some details. I assume your sources are obtained by `fetchurl` or a similar fixed-output derivation. These specify the hash of the output, and the path you have in nix store matches that hash, so it is actually the correct source (considering what you asked of nix).
The typical easy path is to overwrite the hash in `fetchurl` to some incorrect value, e.g. replace some characters by zeros, and nix will then try to re-fetch and print the resulting (mismatching) hash which you can then update. There's also `nix-prefetch-url` to avoid doing fetches twice, etc.