How can we detach a policy from an aws role instance? https://docs.aws.amazon.com/cli/latest/reference/iam/detach-role-policy.html

Pulumi will converge on "desired state" for any resources it owns So if it owns the instance role you would just remove the policy from the source and it would be deleted

yeah unfortunately I'm using an external library so don't quite "own" a handle to the resouce but instead a reference to it's arn.