This is something I see a lot of, and it's lead to a lot of massive throws, not to mention just general ill will and various "negative attitude" reports. I've heard that the PBE Tribunal is staffed purely by Rioters, but that can't be very easy (nor does it appear to be working so well as it does on Live). Since Rioters are the ones staffing the tribunal, I can only assume that the PBE environment is meant to be one filled with upstanding players dedicated to testing and generally being good people.
I have an idea to help destroy the idea that "This is the PBE, you're not going to get banned" and improve the qol of the PBE itself.
Within the community would be upstanding players with the ability to give out higher priority reports. When two or more of these people report a single player within a certain period of time, a warning message is automatically generated, letting the player know that yes, they can be banned, and will be banned unless they clean up their act. If they receive more reports from these special players, then their tribunal report is shifted to a higher priority, allowing rioters to not waste time on reports that don't matter.
The key to this is the automatically generated warnings. With more warnings being distributed, people become more wary of crossing the line. For the special players, they could be selected one of two ways. Players who are upstanding and have been upstanding for a while could either be randomly chosen or could be asked to participate. If they were randomly chosen, they'd have no knowledge that they are participating, and would not change their reporting behavior. If they were asked to participate, then they may not use their reports so frivolously.
Of course, these players would still have no real power over other players, making sure that they could not become tyrannical moderators or something like that.