How can I increase my war score when the enemy is fully conquered?

You do not (despite what you might think), require 100 warscore in order to end the war.

Simply bring up diplomacy with your enemy and select your single wargoal. Whether or not they choose to accept your terms depends on their relative warscore (though 91 is a lot for a single planet, so I assume you would be fine) + other factors, such as relative military strength and length of war (with having a much smaller army increasing their willingness to negotiate, as does war that has been going on for longer times).

If you do not select the war goal, this method will offer white peace instead.

Non-military stations do add to war score if destroyed (they count as a battle), so if that's literally all they have left and can produce no more ships, go hunt them down.