"If they don't leave immediately, what happens? Does an elder come up and ask them to leave?"
Any DF'ed person attending meetings is working toward reinstatement. They are unlikely to stretch the rules given to them since the elders laying down the law are the ones who will decide when they have shown a "humble and repentant spirit" and are worthy of returning.
So the elders may talk to a DF who did not leave promptly. And the DF will tread on eggshells to please these elders. Some elders are overly tough on them because A) they feel it is good to show "tough love" *blech,* and B) they enjoy the power of their unearned authority.
Belligerent DFs will not usually attend the meetings.