Really aluminum foil, but everyone calls it tin foil these days.
Actually, any kind of sheetmetal “hat” on the antenna renders the GPS blind. Of course, NYNY could be cheaping out and using cell phone based tracking instead of GPS, and the tin foil may not be as effective, thats where the wire cutters (known as “dikes” in the trade) come into play.
The solution is real easy. Cut GPS=no transmission. No transmission=no display on Dispatch screen.
No display on Dispatch screen=Dispatcher call: “Unit 1447; Unit 1447, I show you as offline. I guess you don’t get paid for today” ;-)