The protest at which the officer was injured, attended by Mr. Mckesson, was held in Baton Rouge in July 2016 amid widespread demonstrations over police shootings of black men. The officer alleged that Mr. Mckesson had helped incite violence at the protest. That month saw five police officers killed at a march against police shootings in Dallas, and three more killed in Baton Rouge.

This summer, the officer added two more parties to his lawsuit. One was Black Lives Matter Network, Inc., a group associated with the movement. The other was “#BlackLivesMatter,” which, Judge Jackson noted repeatedly in italics, is a hashtag, a marker used on Twitter to flag posts about a similar topic.

In his ruling, Judge Jackson acknowledged that groups and individuals associated with the movement can be brought to court. But Black Lives Matter was an exception, he said.

“Black Lives Matter,” as a social movement, cannot be sued, however, in a similar way that a person cannot plausibly sue other social movements such as the Civil Rights movement, the L.G.B.T. rights movement, or the Tea Party movement. If he could state a plausible claim for relief, a plaintiff could bring suit against entities associated with those movements, though, such as the National Association for the Advancement of Colored People, the Human Rights Campaign, or Tea Party Patriots.

Judge Jackson also chafed at the inclusion of the hashtag, which the officer and his lawyers defined as “a national unincorporated association” in California.

For reasons that should be obvious, a hashtag — which is an expression that categorizes or classifies a person’s thought — is not a “juridical person” and therefore lacks the capacity to be sued. Amending the Complaint to add “#BlackLivesMatter” as a Defendant in this matter would be futile because such claims “would be subject to dismissal”; a hashtag is patently incapable of being sued.

In the end, he criticized the officer and his lawyers for including either.

Plaintiff’s attempt to bring suit against a social movement and a hashtag evinces either a gross lack of understanding of the concept of capacity or bad faith.

Billy Gibbens, a lawyer for Mr. Mckesson, said he was pleased with the ruling.

“DeRay has repeatedly said that he doesn’t endorse violence, and we’re sorry for what happened to the officer, but I think the judge was right that he’s not responsible,” he said.

This case may be resolved, but another, brought before the same judge by a different officer, against Mr. Mckesson, Black Lives Matter, the hashtag, and others, remains, Mr. Gibbens said.