Detecting SPam over Internet Telephony (SPIT) and preventing users from it is a raising concern in the research community. At the time of writing some solutions have been already proposed but, to the authors' best knowledge, no extensive study of prevention effectiveness has been published. This paper presents a modular simulator which was built in order to test SPIT prevention systems' effectiveness that is specifically tailored to deal with upcoming SPIT threat and takes into account VoIP and SPIT-specific configurations and parameters. Making use of the developed simulator, the paper also presents a preliminary study on how the single modules can be optimized by means of the simulator and how multiple detection mechanisms can cooperate in order to achieve higher detection accuracy. Moreover the paper provides effectiveness results showing how the modular approach is the most promising for dealing with the SPIT threat.