"Force post" transactions are when a merchant charges a card without an authorization. It’s literally an unauthorized charge, it’s against payment processor rules, and, unfortunately, it isn’t something that Privacy can block. It's as if the merchant is forcing their way past the authorization normally required, and directly taking money out of someone's account. Merchants use it very rarely, and you won't see any big merchants doing this because you closed a card to cancel a subscription.
Broadly speaking, merchants only resort to force posts when they are faced with the potential for serious loss as a result of fraud. For instance, if you rent a car and don't return it, having a limit on a card or closing the card does not absolve you of the responsibility to return or pay for the car and the merchant has the ability to force post the charge.