Repeated anti-social behaviour, as previously said. If you’re looking for a bloodied blade or smoking gun, this isn’t how we evaluate anti-social behaviour.
For example, leaving a rated Solo Shuffle is a perfectly acceptable thing to do. Real life happens. Disconnects happen. Repeatedly leaving solo shuffles will get you banned for the season.
Another example would be boosting. Queueing 2s with a friend worse than you is a perfectly normal thing to do. If you’re a 3k player and your buddy is 1700 and you wanna help him get to 1800, go for it. If you spend 6 years of your life queueing 10,000 2s games per season, with hundreds of different people, all of which mysteriously trade you gold or paypal you right before you queue up - this is anti-social behaviour and doing it will/should get you banned.
As a final example - ganking. I gank people a lot particularly on my rogue. I also see other people that love ganking people too. Theres a DH from Ravencrest and some fire mage called Ivy from ravencrest too. They gank people a ton. Ganking people is not anti-social behaviour and is not bannable. Camping people is. If you spend your day camping someone, track them down over the course of weeks or months or years, you will be banned for that anti-social behaviour.
So circling back - premades. Queueing with friends is normal and fine. Queueing with 30 of your friends into AV by using discord, chain leaving queues and doing this for years? This is anti-social behaviour and should get you banned.
5 stacking. 5 stacking is fine. You and your buddys wanna queue? Np. You and your buddies wanna queue only as a meta RBG 5 stack comp, with full gear, feasts, flasks, potions, banners and so on, for years straight every day? Anti-social behaviour, ban 'em.
Warn them first, but hit them with the ban hammer if it keeps happening.
“b-b-but, we just wanna have fun!” - fantastic, i love fun too. If you navigate to your PvP menu click ‘rated’ and then click ‘10v10’ you can queue all day with premades and no one will mind.
Alternatively, you can challenge specific premades and queue up a wargame against them - never mind, they removed that feature because it wasn’t used because mysteriously no one wants to play against a premade by choice.