"There is nothing to talk about"
Usually this implies a topic, such as in "There is nothing to talk about this book". In French, we'd prefer to explicitly say what is the topic:
Il n'y a rien à dire sur ce livre.
"Il n'y a rien à dire" feels rather weak in this context. Maybe "Il n'y a rien à en dire", just like @YohannV. said is acceptable, but really feels weak. Usually to insist that there is nothing to talk about, you name that thing. It can even be done like this: "Ce livre, il n'y a rien à en dire."
People who really want to be heard will even say out loud "Il n'y a rien à dire" before saying, very slightly toned down "sur ce livre". The subtlety here is perceptible for native French-speakers.
But as a rule and unless it's really personal (such as emotions), you name the context.
"There is nothing to discuss"
Il n'y a pas matière à discussion.
Il n'y a pas de matière à discussion.
This clearly means that the discussion will lead nowhere and it's a very strong indicator that unless a very fundamental change happens in the topic, this topic shouldn't be brought again. Both forms are accepted and the first might be preferred. Depending on the tone, it can be very polite or very firm.