See, the only problem is, premades are impossible to “police” 
Even if they banned people Q’ing up for bgs as a party, that still would not stop premades. And of course, Blizzard will never ban people playing together.
The more well-known premades like Hydra, Stormpike Militia, Cryptid’s League, etc don’t even Q up as a “party”. Most of the time they coordinate their Q at the same time via third-party comms such as Discord (so that all premade members get into the same bg).
Like I said, it’s impossible to “police” what goes on outside of the game, so premades are not going anywhere.
My own personal “solution” for premades? If I see that I’m up against a premade I’ll usually just leave and take deserter, there’s no point in wasting my time when the deck is stacked against my team (aka fighting against an organized, comms-using premade).
In conclusion, nothing can be done.