There is some indication in this article that credit freezes are not possible in Canada:
"TransUnion Canada does not offer the same type of credit report freeze as ... the U.S., but we do make available fraud alerts," explained John Branham (Branham was a TransUnion spokesperson when he commented, but is no longer in that position).
This statement is supported by a quick review of the products offered by TransUnion Canada - there doesn't seem to be any mention of freezing credit.
As I understand credit freezes, you must be able to freeze your credit with each and every credit reporting agency in order for a freeze to be effective. Otherwise, someone impersonating you to obtain credit will simply choose only institutions who run their checks with the reporting agency that doesn't offer credit freezes.
That same article provides an explanation for why credit freezes are offered in the USA but not Canada:
Americans can order credit bureaus to block or "freeze" third parties -- including creditors -- from viewing their credit file. But no Canadian province or territory has any law that enables such tough credit file protections.
In the United States, on the other hand, it looks like almost all states require that credit reporting agencies allow you to freeze your credit.