Can a court declare someone a vexatious litigant for repeatedly challenging the same will in Ontario?

Yes, in serious and extreme cases. Ontario courts have the power to declare a person a vexatious litigant where they've repeatedly brought proceedings — including repeated challenges to the same will or estate — that are without reasonable basis, that relitigate issues already decided, or that are used to harass or wear down the other side rather than genuinely pursue a legitimate legal claim. This is a significant, relatively rare order, not something imposed simply because someone has lost a will challenge once or is persistent in pursuing a legitimate concern.

Once a person is declared a vexatious litigant, they generally can't start any further court proceedings relating to the estate (or sometimes more broadly) without first getting the court's permission, which acts as a screening step to prevent further unfounded litigation. Courts look at the pattern as a whole — the number of proceedings, whether the same issues keep being relitigated, whether earlier findings were simply ignored, and the toll on the other parties and the court system — rather than any single case in isolation. If you're facing repeated, seemingly baseless challenges to the same will, or you're concerned your own persistence could be viewed this way, get legal advice on how courts assess this pattern before taking further steps.