Staying on topic is very important in debates and discussions. A Red Herring is a statement or question that distracts from the subject at hand. This often happens in a number of ways:
A Red Herring can seem similar to a Strawman, the distinction being a Red Herring changes the topic while a Strawman changes the opponent's argument.
Simply restate what the conversation is supposed to be about and ask them to address it. If another important topic is brought up, offer to table it for the time being and revisit it after the current discussion is completed. If you both decide to change subjects, make it clear to all that the topic is changing.