WASHINGTON—A federal judge banned Roger Stone from using Instagram, Twitter and Facebook over what she said were his repeated violations of a gag order to not discuss any aspects of the government’s case against him.

After a lengthy court hearing on Tuesday, Judge Amy Berman Jackson imposed a complete ban on Mr. Stone’s use of the three major social-media platforms, finding him in violation of the conditions of his release from prison and of her orders.

Judge Jackson didn’t immediate move to hold Mr. Stone in contempt of court, which could open him to further charges or fines, nor did she revoke his release on bond in advance of his November trial. But she suggested she might reconsider such actions in the future.

At issue were several Instagram posts from Mr. Stone that government lawyers contended were in violation of the judge’s order from February and could have an impact on their ability to form an impartial jury.

A lawyer for Mr. Stone, Bruce Rogow, called worries about a fair trial overblown, saying the judge’s order was based “upon a false premise on a danger to a fair trial” and he didn’t anticipate trouble finding an impartial jury. He also asked the judge to consider lifting her gag order entirely.