Don't the cabin crew at the head of each emergency slide stop Pax from taking belongings such as bags down the slide?
The problem is that by that point, the damage is done in terms of time, and trying to extract someone's bag from them is likely to result in more delay if they argue. That's unless you've got a flight attendant who's trained to kick the offender in the nuts, grab his bag and push him down the slide in one easy, fluid motion.