[greymule]

I think you did the right thing in preventing the pitch, just as you would not allow the pitcher to deliver with F5 still in foul territory, returning from chasing a foul ball. I have warned the catcher many times (before the pitch) about being in the box. The most common violation occurs when the batter moves up in the box and the catcher sets her feet in front of the catcher's box to get the mitt right over the plate.

You hold up play when a runner has not yet made it back to her base after a foul ball. You also correct the situation if you see a batter setting up with a foot outside the lines of the batter's box. In the case you mention, if the catcher is set up properly and the pitcher starts her motion, then if the catcher leaves the box you call the illegal pitch.