Not knowing EVE - is it at all possible it was a bot?
Bots in EVE usually stick to safe pursuits like mining; the whole getting blown out of space repeatedly thing would cramp the style of a loot-thieving script. Can't say for sure, of course...
It's a relatively common low-level pirating practice, flipping cans. At the worst you get the loot and maybe told off in local, and at best you get a small-scale fight in supposedly save Empire space.
A lot of the time people who are mining/ratting aren't fitted for PVP whereas the would-be pirate is more than ready for it.
Swipe enough loot and the warning goes away. The players was probably bored or completely ignorant. Did you warn him yourself? A Vexor isn't a huge loss if it's insured, and most Myrmidon ratting builds don't have warp scramblers. Risky, but EVE is all about risk. :)
It's possible that he'd been stealing loot for a while and turned off the warning. Or that he simply thought that since you're ratting, you wouldn't want to fight him.
You should definitely fit a warp scrambler if you have the space. If you blow a loot thief instead of just scare them away with a few shots, they're less likely to try and steal loot from you again.