Upvote:2
You will need to check the terms and conditions of the specific policy. Some insurances apparently allow this. See for example:
https://www.insuremytrip.com/travel-insurance-policies-and-claims/canceling-a-policy/
Single trip medical policies can usually be cancelled any time before departure. While some companies reimburse in full, the majority of them will refund in full less a processing fee.