[Dave Hensley]

Originally posted by Gee
OK, I have learned, from the Great One, that "if a runner legally acquires title to a base, and the pitcher assumes his pitching position, the runner may not return to a previously occupied base" (OBR 7.01).

If the runner attempts to return the umpire should warn him and if he persists should be called out. (JEA, J/R, Fitzpatrick and Childress).

There is nothing in JEA, at least, about "warning" the runner to return. JEA simply and cleanly instructs to call the runner out. From the applicable Professional Interpretation:

When the pitcher assumes his position on the rubber prior to delivery, no runner may return to a previously
occupied base. If he attempts to do so, the umpire shall call "time" and declare him out.
Originally adopted to eliminate a trick play and unorthodox strategy, this rule accomplished its purpose. Such shenanigans are unheard of in the modern game.
Umpires should be alert and declare out any runner who should return to his previous base after the pitcher has
assumed his position on the rubber. This could most logically happen when the runner felt that he "left too soon" on a tag-up and would attempt to return before an appeal was made on him.