It's against federal law to disobey a crewmember on board an aircraft. Once that door closes, if you want off, it is at the captain's descretion. He never has *any* obligation to let you off just because you want him to, even if you're still at the gate. They can't be charged with kidnapping because the law gives them that authority.
On older aircraft, there's basically a hook that hooks the top of the slide to the airframe with the door closed, opening the door without releasing it will activate the slide to deploy. On newer ones it's a lever in or next to the door itself.
So yes, the FARs do prevent you with just exiting on your own. This falls under the laws the same way as if you tried to open a door while the aircraft was taxiing out for departure.