Oh, the RNC absolutely can keep Trump from the nomination - and they will. But they know, if they do and pay the price, that the end is near.
So they prefer the media and Nikki Haley, or the courts, to do their work for them.
But the RNC is a private organization, they can do whatever they want with their nomination, and they probably will.
If the RNC refuses to name Trump as the nominee after he wins the primaries they are done as a national party. At least 10% we likely closer to 30% of their base will walk away from them permanently.