A system and method for controlling data communications between a server and a client device, such as a mobile device. Embodiments relate generally to a technique where stop data is provided to the client device. This stop data can be transmitted (e.g. by the client device) to the server. When processed by the server, the stop data indicates to the server that at least some of the encrypted data received by the client device from the server was not decrypted using the second key (e.g. as may be the case when the second key has been deleted). Upon receiving the stop data, the server may, for example, withhold the transmission of data encrypted with the first key to the client device until the second key is restored on the client device. In one embodiment, the stop data is provided to the client device in an encoded (e.g. encrypted) form.