This just happened at my club yesterday while i was RO'ing a competitor who routinely starts prematurely. He's selective about when he jumps, but always during a classifier. On this occasion, it caught me off-guard and i let it slide, thus giving him a big advantage on a difficult PAR time stage. On the second string of that same classifier, he jumped again, and i stopped him... and gave him a warning.
I differentiate creeping vs. false start as... when a competitor false-starts accidentally and stops himself, i consider that a false start and will give them another go. But if he doesn't stop himself, i feel it was intentional and they should be stopped/procedural'ed.