People from various hacking collectives have tried for several months to block social media accounts that spread propaganda and attempt to recruit fighters for the Islamic State, but those campaigns gained a new energy on Twitter after the Paris attacks.  Mark Wallace, the chief executive of the nonprofit Counter Extremism Project, supports the reporting and blocking of accounts en masse. “When the puppet pops back up, in the context of social media it’s not as strong a puppet,” Mr. Wallace said. “You’re weakening that person’s presence on Twitter with each time you take them down.”