Can I cancel a designation of matrimonial home that was registered by my ex wife?

Yes. You indicate the designation was registered, and we will assume you mean formally registered at the appropriate land registry office.

Under the Ontario Family Law Act one or both spouses can designate and register property as a matrimonial home (in section 20(1) and (3)).  There are special rules as to the legal effect when—as in your situation – only one spouse has done the registering (in section 20(5)).

However the designation can also be cancelled in one of several ways, under the Act’s clauses 20(6)(a) through (d). There is provision made for depositing or registering a cancellation (see clause (a)), but this only applies if the cancellation has been executed by the person who made the original designation in the first place (i.e. your ex-wife).

Barring that, clause (b) allows you to register a divorce decree, while clause (c) allows you to register a court order cancelling that designation.  (And clause (d) calls for proof of death to be registered, if applicable).    In your case, the easiest way would be to simply register your divorce decree.