Have had a passenger tell us he wanted to get off, just before we shut the doors. We had a slot. His reason? his girlfriend hadn't made the flight. We told him it wasn't possible for him to get off at this point (well, we thought it was worth a bluff as otherwise we'd have a big delay whilst we found his bag and got a new slot). His reaction? "Oh ok then" and he went and sat back down!!
If the person was mad or ill then would probably off-load. Otherwise no way.