Narrowband-Internet of Things (NB-IoT) is a relatively novel Low Power Wide Area Network (LPWAN) radio technology used to deploy Internet of Things (IoT) infrastructures at scale. It is important that such deployments are resilient to attacks. In this work we describe how interference on the NB-IoT synchronisation signals - the initial communication steps - can be used to implement an effective Denial of Service (DoS) attack. Interference with the synchronisation prevents communication and may also allow an attacker to force a device to connect with a specific base station.