SBC could stop in conference transfers
I suggested that on the SBC board a good while ago and they all balked at it. Said they'd rather keep the talent in the SBC. I don't like the idea of having people that know your playbook now playing against you for the remainder of their eligibility. It doesn't happen often enough for "keeping the talent" in the conference to seem like a thing. but even one gives that team an advantage over a conference mate in terms of knowledge of the team and their playbook.
And it would be legal if the conference adopted it as a bylaw. If someone enters the portal the other teams just don't accept them into their school. Nothing illegal about that. Especially with NIL, you don't want conference mates outbidding you for your players. It's dirty pool.