Yes, you can do this directly through the app. If you feel like someone has posted a listing that goes against community standards in any way, or is being abusive or harassing you (or other users), you may choose to block them. Simply go to their profile, tap on the three dots in the upper right corner, and select Block or Report. For repeat offenders, it is best to leave a review of their behavior. You can do this by opening your 1-on-1 conversation and tapping on the Rate button. In addition, you can also unblock someone if you decide to patch things up.