Catcher in catcher's box prior to LBR?

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A non-veteran ump here, and I haven't seen (recognized) too many LBR violations. I did want to clarify for Dave though, at least with my understanding of the rule. If the runers had returned to the bases, then left again, they would be out. Or, at least the first one would be if that totally kills the play, and we could only ring one of them up. If they were returning to the bases, they can stop once (one-thousand-one) then have to immediately go one way or the other. Now, here is my question:

If a runner stops at a base, and pitcher does not have control of the ball within the circle, can the runner leave the base at that point? I seem to remember some statement in the rule book (ASA) that would prohibit this, but don't have the book with me.

thanks,