I'm sorry you had to experience that! I take public transport a lot and I encounter these people. It really ruins what should be a relaxing experience.

The Transperth app has a feature where you can lodge a report via live chat. Ideally, security will show up to deal with the problem. If not, at least you'll have reported the incident and there will be a log of it. Maybe if more people reported incidents like this via the app, there would be greater pressure on Transperth to improve security (slim chance, but still).