The Justice Zone is a sort of energy field surrounding a space prison visited by the crew in "Justice". If anyone attempts a crime within the Justice Zone, the consequences happen to the perpetrator instead of their victim. Lister learns this hard way when he tries to set fire to Rimmer's bedsheets and instead sets fire to his own jacket. The thinking is that once the prisoners have served their sentences, being forced to live within the law will have become second nature
Other examples include if a prisoner steals something, something of theirs goes missing. If someone attacks someone else, they feel the pain of their own blows and are shot/stabbed by their own weapons. In a deleted scene, "Litter Birds" drop their "cargo" on any who litter in the park (which Lister did accidentally).