Multiplayer in Watch Dogs can be turned off
When the lead game designer Danny Belanger talked to Polygon he revealed that “All the online activities are just part of the menu. They’re a choice. But if someone is not into that, they can totally turn it off.”
It seems that Watch Dogs has also something called “the shield” that helps the players not to be invaded by hackers. Belanger said that a player cannot be spammed unless he becomes aggressive or he starts going into other players games. In the end it’s just a player choice.