Jamie Crooks, Alex Gogolev, Steven Hodges and Joe Hicketts scored for the Royals (29-19-4) while Shea Theodore had the only goal for the Thunderbirds (19-30-5).
The Royals opened the scoring when Crooks picked up a loose puck after a faceoff and snapped it past Thunderbirds goalie Brandon Glover.
Gogolev made it 2-0 with less than a minute left in the first, scoring from a perfect goalmouth feed from linemate Brandon Magee.
Thunderbirds defenceman Theodore made it 2-1 early in the second on the power play, blasting a rocket from the point that beat Polivka cleanly over his shoulder.
Victoria restored the two-goal lead on a two-on-one. Glover kicked out the initial from Hodges' shot but it went in off a defender's skate.
Hicketts added an empty-netter in the final minute of the third.
Polivka made a number of key saves throughout the game, stopping all nine shots in the third period for the win.
Glover saved 20-of-24 shots for Seattle.
Gogolev left the game early in the third period with a lacerated left foot.