Aggregate Computing is a promising paradigm for coordinating large numbers of possibly situated devices. It is used, in particular, in scenarios related to the Internet of Things, smart cities, drone fleet coordination, and mass urban events. Currently, however, little work has been devoted to study and improve security in aggregate programs. Existing works only focus on application-level countermeasures, typically introducing trust metrics to detect misbehaving devices. Those security systems work under the assumption that the underlying computational model is respected; however, so-called Byzantine behaviour violates such assumption. In this paper, we discuss how Byzantine behaviours can hinder an aggregate program, and exploit application-level protection for creating bigger disruption. We discuss how the Blockchain technology can mitigate these attacks by enforcing behaviours consistent with the expected operational semantics, with no impact on the application logic.