How should one handle hijackers? Shoot them!!
Ah but that is cruel, not legal, and unjust... But they are hijackers!
So what is the next best option. Get off the ride, let them go free, but not let them take us with them.
So we cancel the event.
SJW aka Social Justice Warriors, or similar pejorative terms are essentially hijacking an organization's function to provide a different message, to redirect a organization into a different direction.
Hijacking comes in many forms. Archery club being used by drone flyers. Just do not displace archers, or they will use your drones for targets.
Atheism is a vast tent of a simple idea, there are no gods. After that we can look at the arguments that there are no gods, perhaps the damages religions have done to humanity, the biases of religions, and figure out how to replace the few things that religions do well; build community, perhaps extended social functions... Much religion has been already replaced by mass media. Moral and ethics, well, some religion provided some, but that has been partly replaced by national law systems, and common morals communicated by mass media. Morals and ethics are not dependent on religion, as they prescribe the morals of iron age people; our society is now troubled by fraud, computer crime, gambles sold as investments, Grifters, Ponzi schemes Charlestons, bullshitters, reality show host, and the like, not of the things of biblical times. After we cover those few items, where does that leave atheist organizations? Expanding to take in more unbelievers, and bring them up to speed, whatever that means. And there we run out of works, for without works, our no god philosophy is dead. So along come the SJW's, and those who would hijack our organizations on to new philosophies that have nothing to do with the original no god cause.
Many things we can do in private, we cannot do in public, as some nosy busy body will stick there nose in where it does not belong, and start pushing there biassed values onto others. Likewise insiders in the organizations can try to impose there own agenda onto the organization, and the result is always destroying/reducing the organization, or redirection of the objectives. So now we see the atheist organizations get sidetracked into LGBT, gay rights, inoculation advocacy, climate change or climate change denier, for or against fracking, ethics development, moral authorities, abortion or pro-life... any of long list of social issues.
So what are the limits of the organization? What is the mission statement, or the objectives of the said organization? If the speakers are pushing topics outside of the stated purpose, the participants, or volunteers may feel it was a waste of their time, subject beyond that which was advertised... not on topic... and will not be involved any more.
This is what will bring down any volunteer operation, if there is no value, as judged by the volunteers aka participants. If members in the administration get off topic, the organization changes direction, and may go where the members do not wish to go. That is the risk in any volunteer organization, is there enough interest, and can we hold interest? Can we replace members fast enough if retention is not high enough?
A clear and inclusive mission statement, with some exclusion clauses if require, or written objectives, are the way to direct an organization, is the way to keep organization healthy.