A mobile agent system consists of mobile agents, each of which can transport its state and code to any computing host in the system. The system is able to perform dynamic network reconfiguration and user customization to keep it flexible and effective. A mobile agent system can be theoretically depicted by the mobile actor model, in which a mobile actor is an autonomous computing entity and communicates with other actors by using asynchronous messages. Automatic distributed actor garbage collection, one of the important issues of lifetime management of mobile actors, can help high-level programming and efficient resource management in such a mobile actor system. Therefore, it is beneficial to design an intelligent management strategy to automatically detect and collect mobile actor garbage. In this paper, we propose a mechanism for mobile actor garbage collection using a novel conservative snapshot-based algorithm. Compared with prior similar work, the proposed mechanism is more flexible because it does not assume FIFO communication and any network topology. Moreover, it can perform actor garbage collection in a partial set of computing nodes, and thus can reduce the synchronization overhead while taking a global snapshot on a particular distributed application. In addition, the proposed mechanism does not restrict mutation operations such as actor creation and actor migration while taking a global snapshot. We have conducted experiments to evaluate the overhead of the proposed mechanism, and the results showed that the proposed mechanism is practical.