On the app there are no reasons, but on the website when you choose to hide a thread you will be given a box asking your reason. If you just wish to hide something, you choose 'personal reasons.' On the app, just report and it's the same thing. Reporting a post only hides that specific post. If you wish to hide that specific poster's posts forever, you can mute them from the website by clicking their name and hitting 'mute,' and on the app by going to their profile page, then going into the options and choosing 'mute.'