Ottawa created two chances early in the second period, including a three-on-two rush where Sarah Wozniewicz shot over the net.
Montreal broke the scoreless tie when Roque's shot from near the boards deflected off Ottawa defenseman Rory Guilday's stick and past goaltender Gwyneth Philips.
Wozniewicz later hit the right post, and Emily Clark failed on a breakaway against Desbiens.
Ottawa started the third period aggressively but could not generate clear chances.
Montreal doubled its lead during a penalty to Poulin for obstruction when Roque beat Brianne Jenner and slipped a backhand past Philips.
Prior to that, Jenner had been stopped twice by Desbiens on an Ottawa power play.
Maggie Flaherty made it 3-0 with a one-timer from the blue line that changed trajectory before beating Philips.
Lina Ljungblom scored the final goal with less than five minutes left after capitalizing on a defensive turnover by Jenner in front of the Ottawa net.
The loss marks Ottawa's second consecutive finals defeat.
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Meanwhile, the Montreal Canadiens are competing in the NHL Stanley Cup playoffs, having advanced to the Eastern Conference Finals against the Carolina Hurricanes.
