For those of you who are indeed Canadian Citizens, let's go over the Canadian Charter of Rights and Freedoms, Subsection 6(1):
Every citizen of Canada has the right to enter, remain in and leave Canada.
And there's no additional conditions on this. So if anyone, including Air Canada, refuses your right of leaving Canada for any reason - e.g., without round-trip ticket - you may want to seek legal actions.
At the mean time, get a round-trip ticket and leave.
