If you're playing on PC, you can simply block the game's executable from accessing the Internet with your firewall. To do this in Windows Firewall, run the firewall GUI by typing "firewall" into the start menu and clicking on the appropriate item, navigating to Outbound rules, clicking on New rule..., choosing Program, This program path and choosing the Dark Souls 2 executable, Block the connection, selecting all domains and giving the new rule an appropriate name. If you wish to enable multiplayer again, open this rule in the firewall GUI and change it to Allow the connection, and vice versa if you want to disable multiplayer. If you're using a 3rd party firewall instead of the built-in firewall, you likely know how to do the same in your firewall of choice, or you may google for instructions.
If for some reason you cannot do this, or you're playing on a console, you can always unplug your device or disconnect it from WiFi.