There's plenty of things they could do. Their inaction is aiding it plenty.
-affecting scholarship limits/academic standing of the program based on players leaving early
-maybe make it so scholarships are "locked up" for two-years minimum
-affecting future postseason eligibility
-way closer scrutiny on the academic standing of those 1'n'doners in their second semester
-probably plenty of other things that I haven't thought about
Instead, they stand by. It's a complete mockery of their Platonic ideal of a student-athlete, sure, but they love having those Rivals stars in their programs for a year, love basking in the reflected glow of the NBA and the draft, and love ESPN covering *their* future de facto properties before they come to the NBA with up-'n'-coming stories, draft intrigue, projections, and as a AAA for the association. CFB is the same way for the NFL.
They'd never get the same coverage on some star senior who is probably going to be playing in Europe or coaching next year (i.e. Georges Niang) than they will somebody who is 3-4 years younger, not as good/polished of a player, but likely playing against Lebron next year, which means less $$$, so they'd never do it.