Those who disagree will be treated like anyone else. If a person or group decided to start promoting a return to capitalism, or some other class-divided social form, they would be free to do so. If however, a person or group, was damaging society (beating people up, or blowing up buildings, etc.) then society will take appropriate action against them.

Freedom must include allowing disagreement with the status quo and spreading unpopular ideas, but freedom does not include hurting people or destroying the common wealth of humanity. Exactly what methods a future socialist society will democratically choose to use, if people need to protect themselves, are beyond the ability of the World Socialist Movement to predict, but one can expect that those methods will be more humane and less dictated by blanket policies than the methods used today.