Donald Trump isn’t exactly a voice for social media accountability, so it came as a welcome surprise on Tuesday when Mike Flynn, Jr. was fired from the Trump transition team after spreading a bogus online theory linked to a Washington shooting incident.

Mr. Flynn is the first transition team member to get the boot for trolling. He is the son and close aide of Lt. Gen. Michael Flynn, Mr. Trump’s pick for national security adviser, who shares his love for social media conspiracy theories. General Flynn’s son was one of thousands of online trolls “investigating” #Pizzagate, a wild chain of false claims imagining that Comet Ping Pong, a Washington pizzeria, is a front for a child abuse and trafficking ring led by Hillary Clinton and John Podesta, her former campaign manager.

On Sunday, Pizzagate took a more serious turn when a gunman turned up at Comet with an assault-style rifle to “self-investigate” the phony claims. The suspect, Edgar Welch of Salisbury, N.C., spent 45 minutes inside the restaurant, firing at least one shot in an effort to find imprisoned children.

After the gunfire inside Comet, Mr. Flynn continued to spread the Pizzagate claims on Twitter, saying “Until #Pizzagate proven to be false, it’ll remain a story.” When Jake Tapper, a CNN reporter, called him out on his behavior, Mr. Flynn attacked him on Twitter, tweeting that Mr. Tapper was “trolling the family of a highly decorated retired military officer.”