A.I. Absolutely will not surrender!

[Stone Mill]

Do some kind of extremely small scale bombardment (with a frigate with one weapon, for example)to whittle them down until the population ratio triggers his (actually, Queen Triana's) surrender.

Check for sure that they did not colonize any moons (pesty buggers).

Concurrently continue to grow your score as well.

Also, if you have not already, you may also wish to propose a trade (your colonization tech for theirs); they have a good chance of accepting this, even at war.