I agree with JimmyT, mediation in NSW is not compulsory, so attending with a vexatious litigant would be pointless and a waste of time. I would also do everything by the book. Certainly defend all claims at the Tribunal in a respectful manner. Make sure you document everything, keep all evidence in case of future litigation and try demand costs from the litigant through the Tribunal. It is very difficult to ‘stop’ a vexatious litigant, just keep on top of the situation at all times and don’t become complacent.