At some point you will either have to escalate or give up. Some people won't be convinced, and can't be badgered into obeying you, and they know that you have no recourse at all as long as they remain seated and ignore you.

If the local authority (flight attendants) won't back you up, you are at the discretion of whoever is hogging your seat. Any display of power over the other usually amounts to potentially being able to resort to violence if necessary to back up your power. The person in the seat know full well that you cannot do that without being forced to leave the plane, so they have effectively nothing to fear from you.