"Force post" transactions are when a merchant charges a card without a prior authorization. Unfortunately, it isn't something that Privacy can block.
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.
If this happens to you, contact us at support@privacy.com and we'll work with you to try to resolve it.