which device would be needed to use on the masternaut system?

Founded in 2002, Masternaut was one of the first companies to use telematics — the integration of telecommunications and information technology– to provide a web-browser based vehicle tracking system. As Europe’s largest and most trusted vehicle tracking system provider, Masternaut vehicle tracking systems have been the choice by many prestigious organizations.

In their simplest form, Masternaut vehicle tracking systems combine GPS(Global Positioning System) technology with GPRS(General Packet Radio System) technology. GPS, as we know, uses satellites to accurately track a vehicle’s location; while GPRS transmits the location data, and optionally other data pertaining to the vehicle’s use to Masternaut’s data center in real-time.
In this case, a GSM GPRS GPS jammer is recommended to jam such Masternaut tracking device:
http://www.jammer4uk.com/gps-and-cell-phone-jammer-j220b-p-17.html

In addition, sometimes WiFi frequency may be also involved in a Masternaut system, because Masternaut vehicle tracking systems optionally allows company drivers to carry a WiFi-enabled tag which interacts with the vehicle tracking system and authenticates them as the authorised driver of a particular vehicle.
Then, to completely block this kind of Masternaut system, you can use a GSM GPS WiFi jammer or even a GSM 3G 4G WiFi GPS jammer to be on the safe side:
http://www.jammer4uk.com/portable-cellphone-gps-wifi-jammer-j240bpro-p-9.html
http://www.jammer4uk.com/portable-cell-phone-jammer-3g4gwifigpslojack-8-powerful-antennasband-control-switch-p-49.html