How to disable online in Dark Souls 1 while keeping Steam online?
Well apparently Dark Souls goes through Steam for multiplayer because of how it used to use GFWL. According to this post, you would need to block port 3074 to block off multiplayer.
A pretty simple and generic way to do that is to open the Windows firewall UI (or any other firewall you have), add the executable and deny it all connections. Do it for both inbound and outbound connections and the game just won't be able to use the internet.