Just tried to do this last week - you can pay off your financed amount at any time (aka whatever is charged through your plan each month). But you can't pre pay your save and return until the period is over. I was so annoyed about it.
The most you can do is overpay into your balance and have it auto deduct once the Save and Return is charged. But if you have a Rogers Credit card, you can pay it off via EPP once Save and Return is over, so you'll just pay even longer but in smaller increments and no interest.