The police working with Tesla would be wise to leave some bait-cars around with extra cameras and nearby arrest teams to gather evidence to prosecute the gang(s) doing this.
What do they do about software updates though? They would probably have to break the firmware to if they still want any updates.
With the Model 3 hitting a much wider consumer market, you'll see a lot more interest in ways to get around Tesla.
I am actually expecting you may also see more lawsuits regarding things like software activation requirements vs. holding the title to the car and such. Tesla treats these cars like they're still Tesla's property, and if anything I'm shocked it hasn't been tested in court more already.
Besides there being many more of the latter they also have special legal aspects in the form of all kinds of consumer protections.
Most Teslas on the road are still under warranty, with Tesla footing the bill for repairs. Once all of those second and third owners of Model Ss start needing out-of-warranty repairs, then I expect we'll start seeing some real heat.
Foil backed foam won't necessarily block a cell signal - heck, there are entire buildings insulated with the stuff and it has little effect.