The Low-Power Wide-Area Network (LPWA) has already started to gain a notorious adoption in the Internet of Things (IoT) landscape due to its enormous potential. It is already employed in a wide variety of scenarios involving parking lot occupancy, package delivery, smart irrigation, smart lightning, fire detection, etc. If messages from LPWA devices can be manipulated or blocked, this will violate the integrity of the collected information and lead to unobserved events (e.g., fire, leakage). This paper explores the possibility that violates message integrity by applying a reactive jamming technique that disrupts a Long Range Wide Area Network (LoRaWAN) network. As shown in this paper, using low-cost commodity hardware based on Arduino platform, an attacker can easily mount such an attack that would result in completely shutting down the entire LoRaWAN network with high probability. Several countermeasures are introduced to reduce the possibility of jamming attacks.