It should be something like this:
{%CurrentUser.ReferringObjects.Subscriptions.Filter(SubscriptionGatewayID == 1 && SubscriptionTemplateID == 123).Count|(identity)GlobalAdministrator%}
Basically you need to check if a record exists for given user and a given template (may be gateway if you have your custom one) in Notification_Subscription table. So if the count greater than 0 - hide the widget/form. I really dont know your setup I am just try to give you an idea :)