If a person is on the plane and the staff ask the person to leave as there are not enough seats or they are causing a disruption, The person must leave. End of story. If they do not comply, The company has a right and obligation to call the police or security to escort the person off the plane and possibly arrest them for trespassing.
Airlines across the world overbook almost all of their flights in order to ensure that all of the seats are booked, which is most cases leads to people being asked if they wish to leave the plane for a sum of money. However, in the case of a completely overbooked flight, the airline should be responsible for finding a replacement immediately to ensure that all customers are able to leave on their flights on time. On top of this, the airline does not have the right to forcibly drag someone from a flight which they have paid to be on if they have done nothing against the law.