Look back rule with a walk

[Dakota]

This subject came up during idle chit-chat between umpires at a recent tournament I called.

ASA rules.

Situation: Base on balls; F2 immediately throws back to F1, who has the ball in the circle before the BR gets to 1B. F1 is mostly ignoring the BR - clearly not making a play. BR rounds 1B, stops, and immediately returns to 1B.

Question: How do you distinguish between "rounding" 1B and committing to 2B?

Many aggressive runners will "round" quite a bit - 3-4 steps or more, taking a wide swing. Usually, these runners are sprinting to 1B, so they have some momentum when they reach the base. One of the blues in the discussion said he would ring them up if they took more than a step. I, on the other hand, looked for more than just steps. If the runner seemed to be taking several steps to slow to a stop, and made no other attempt toward 2B, I'd allow that.

What do you do? How do you enforce the look back rule in this situation?