They can stop it very, very easily. At some point, without an anti-trust exemption, CFB needs political help to reduce access. Much of the move from 2 to 12 in recent years has been for providing more post season opportunity optics as it has been increasing money. CFB is the only NCAA sport with so few schools routinely represented in the post season and that raises eyebrows.
The NCAA has tried for decades to get the NFL's antitrust exemption and if they couldn't do it I guarantee you that the B1G/SEC can't do it on their own. The only way to shake it more is to break away fully from the NCAA for a single sport. Conferences would kind of cease to be and it would move to this organization. But even then, due to not having the exemption, the organization would need to offer a path to joining or these universities would start to be treated as businesses.
So, having 6 autos isn't frowned upon by the SEC/B1G even if they say "it should be the best 12" because by allowing the autos they stay out of federal hearings on their financial structure. And they would, once states lost access, those politicians would start banding together and it would be a huge mess.
On top of that, it doesn't really gain them anything. The B1G and SEC are already going to get 3 each in, they're going to restructure the financials so the B12 windfall of the CFP doesn't occur anymore, you'll get paid by the number of teams you have in your conference, for one, and, likely, performance over time, as another.
In short, their battle for a 4th team, just isn't worth the fight.