So in theory, if Ralph Klein (for example) introduced a bill discriminating against gays, the PM could simply order the LG of Alberta to withold royal assent, and thus "reserve" the bill. The PM also has the power to undo bills already passed, for a year after the passage of said law.
The relevant portions of the Constitution Act, 1867 are Sec. 55-57, 90:
55: Where a Bill passed by the Houses of the Parliament is presented to the Governor General for the Queen's Assent, he shall declare, according to his Discretion, but subject to the Provisions of this Act and to Her Majesty's Instructions, either that he assents thereto in the Queen's Name, or that he withholds the Queen's Assent, or that he reserves the Bill for the Signification of the Queen's Pleasure.John A. MacDonald, our drunken first PM, was probably the champ when it came to annulling and reserving bills. But after his passage from the political scene, the power fell in to disuse. Like the notwithstanding clause, there's little chance it will ever see the light of day federally.
56: Where the Governor General assents to a Bill in the Queen's Name, he shall by the first convenient Opportunity send an authentic Copy of the Act to One of Her Majesty's Principal Secretaries of State, and if the Queen in Council within Two Years after Receipt thereof by the Secretary of State thinks fit to disallow the Act, such Disallowance (with a Certificate of the Secretary of State of the Day on which the Act was received by him) being signified by the Governor General, by Speech or Message to each of the Houses of the Parliament or by Proclamation, shall annul the Act from and after the Day of such Signification.
57: A Bill reserved for the Signification of the Queen's Pleasure shall not have any Force unless and until, within Two Years from the Day on which it was presented to the Governor General for the Queen's Assent, the Governor General signifies, by Speech or Message to each of the Houses of the Parliament or by Proclamation, that it has received the Assent of the Queen in Council.
90: The following Provisions of this Act respecting the Parliament of Canada, namely, — the Provisions relating to Appropriation and Tax Bills, the Recommendation of Money Votes, the Assent to Bills, the Disallowance of Acts, and the Signification of Pleasure on Bills reserved, — shall extend and apply to the Legislatures of the several Provinces as if those Provisions were here re-enacted and made applicable in Terms to the respective Provinces and the Legislatures thereof, with the Substitution of the Lieutenant Governor of the Province for the Governor General, of the Governor General for the Queen and for a Secretary of State, of One Year for Two Years, and of the Province for Canada.
To my knowledge, none of these sections of CA1867 have ever been ammended.
(Whew! That Political Science degree pays off!)