No, currency contract requires both sender and receiver to be in recipients list. Sender or recipient could implement apply_currency_transfer to auto reject every transfer.
An exchange would only block transfers that exceed user's balance with exchange contract, meanwhile other contracts may block all incoming transfers that they do not expect.
Ok I think I got it, it would be up to the contract to make sure any required
recipientshook cannot be abused by a third party and in thecurrencycase it's not a problem if either of the parties would like to stop the transaction from happening.That's really powerful! I can't wait to try this stuff out on the testnet :)