Can a person refuse to undergo a capacity assessment under Ontario's Substitute Decisions Act?

Generally, yes. A person has the right to refuse a capacity assessment under the Substitute Decisions Act in most circumstances, since the Act is generally built around respecting individual autonomy unless and until a proper legal process establishes otherwise. An assessor cannot simply show up and compel someone to participate against their wishes in the ordinary course.

That said, refusing an assessment doesn't necessarily end the matter if family members or others still have genuine concerns about someone's ability to manage their property or care for themselves. Where a court application for guardianship is already underway, or where circumstances allow the Public Guardian and Trustee or another party to pursue other legal avenues, a refusal can lead to the question being addressed through the court process instead, potentially without the benefit of a formal assessment supporting either side. A person facing pressure to undergo an assessment they don't want, or a family concerned that a loved one is refusing an assessment they believe is genuinely needed, should get legal advice about what options exist given the specific situation, since the right approach depends heavily on the circumstances.