Eternal Demon's idea is solid. As I read the first post, I was already thinking
"Can't they make a smurf account and link it to that?" but making them have a level 30 account is a good idea. It also increases the quality of the testers, as a level 30 would at least have intermediate understanding of game mechanics, skin testing and item building to test the balance of new champions.
Making them be level 30 makes them less inclined to behave badly. As for the people where they have a level 30 as another smurf - well, this system concept is really the lesser of many evils. And if they are banned, they will have to link a new level 30 when registrations re-open. They would either stop being toxic or eventually lose all of their accounts.
Also, it may be off-topic, but I think to keep players focused and less toxic, a number of 'objectives' for the PBE listed in the in-game client would really help people know exactly what PBE's goal is, and motivate them to test, rather than to just play with skins and pretend to have VIP access to future content.