In Mobile Ad hoc NETworks (MANETs), certification systems play an important role in maintaining network security because attackers can freely move and repeatedly launch attacks against different nodes. By adopting certification systems, it becomes possible to exclude identified attackers from the network permanently by revoking the certifications of the attackers. A simple way to identify attackers is to collect information on attackers from nodes in the network. However, in this approach, it is difficult to differentiate valid accusations made by legitimate nodes from false accusations made by malicious nodes. In addition, the amount of traffic in order to exchange information on attackers and the necessary time to gather the information increases as the network size becomes larger. In this paper, we propose a certificate revocation scheme which can revoke the certification of attackers in a short time with a small amount of operating traffic. By clustering nodes and introducing multi-level node reliability, the proposed scheme can mitigate the improper certificate revocation due to false accusations by malicious users.